THE WORKS
OF
FRANCIS MAITLAND BALFOUR.
VOL. I.
Memorial Edition.
Cambridge:
PRINTED BY C. J. CLAY, M.A. AND SON,
AT THE UNIVERSITY PRESS.
[Illustration: Sketch of Francis Maitland Balfour]
Memorial Edition.
THE WORKS
OF
FRANCIS MAITLAND BALFOUR,
M.A., LL.D., F.R.S.,
FELLOW OF TRINITY COLLEGE,
AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF
CAMBRIDGE.
EDITED BY
M. FOSTER, F.R.S.,
PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;
AND
ADAM SEDGWICK, M.A.,
FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.
VOL. I.
SEPARATE MEMOIRS.
London:
MACMILLAN AND CO.
1885
[_The Right of Translation is reserved._]
PREFACE.
Upon the death of Francis Maitland Balfour, a desire very naturally arose
among his friends and admirers to provide some memorial of him. And, at a
public meeting held at Cambridge in October 1882, the Vice-Chancellor
presiding, and many distinguished men of science being present, it was
decided to establish a 'Balfour Fund' the proceeds of which should be
applied: firstly to maintain a studentship, the holder of which should
devote himself to original research in Biology, especially in Animal
Morphology, and secondly, 'by occasional grants of money, to further in
other ways original research in the same subject'. The sum of £8446 was
subsequently raised; this was, under certain conditions, entrusted to and
accepted by the University of Cambridge; and the first 'Balfour student'
was appointed in October 1883.
The publication of Balfour's works in a collected form was not proposed as
an object on which part of the fund should be expended, since his family
had expressed their wish to take upon themselves the charge of arranging
for a memorial edition of their brother's scientific writings. That
edition, with no more delay than circumstances have rendered necessary, is
now laid before the public. It comprises four volumes.
The first volume contains, in chronological order, all Balfour's scattered
original papers, including those published by him in conjunction with his
pupils, as well as the Monograph on the Elasmobranch Fishes. The last
memoir in the volume, that on the Anatomy and Development of Peripatus
Capensis, was published after his death, from his notes and drawings, with
additions by Prof. Moseley and Mr Adam Sedgwick, who prepared the
manuscript for publication. To the volume is prefixed an introductory
biographical notice.
The second and third volumes are the two volumes of the Comparative
Embryology reprinted from the original edition without alteration, save the
correction of obvious misprints and omissions.
The fourth volume contains the plates illustrating the memoirs contained in
Vol. 1. We believe that we are consulting the convenience of readers in
adopting this plan, rather than in distributing the plates among the
memoirs to which they belong. To assist the reader the explanations of
these plates have been given twice: at the end of the memoir to which they
belong (in the case of the Monograph on Elasmobranch Fishes at the end of
each separate chapter), and in the volume of plates.
All the figures of these plates had to be redrawn on the stone, and our
best thanks are due to the Cambridge Scientific Instrument Company for the
pains which they have taken in executing this work. We are also indebted to
the Committee of Publication of the Zoological Society for the gift of
electrotypes of the woodcuts illustrating memoir no. XX. of Vol. 1.
Several photographs of Balfour, taken at different times of his life, the
last shortly before his death, are in the possession of his relatives and
friends; but these, in the opinion of many, leave much to be desired.
There is also a portrait of him in oils painted since his death by Mr John
Collier, A.R.A., and Herr Hildebrand of Florence has executed a posthumous
bust in bronze[1]. The portrait which forms the frontispiece of Vol. 1. has
been drawn on stone by Mr E. Wilson of the Cambridge Scientific Instrument
Company, after the latest photograph. Should it fail, in the eyes of those
who knew Balfour well, to have reproduced with complete success his
features and expression, we would venture to ask them to bear in mind the
acknowledged difficulties of posthumous portraiture.
Footnote 1: In possession of the family. Copies also exist in
the Library of Trinity College, and in the Morphological
Laboratory, at Cambridge.
TABLE OF CONTENTS.
PAGE
PREFACE i
INTRODUCTION 1
1872
I. On some points in the Geology of the East Lothian
Coast. By G. W. and F. M. BALFOUR 25
1873
II. The development and growth of the layers of the
blastoderm. With Plate 1 29
III. On the disappearance of the Primitive Groove in the
Embryo Chick. With Plate 1 41
IV. The development of the blood-vessels of the Chick.
With Plate 2 47
1874
V. A preliminary account of the development of the
Elasmobranch Fishes. With Plates 3 and 4 60
1875
VI. A comparison of the early stages in the development
of Vertebrates. With Plate 5 112
VII. On the origin and history of the urinogenital organs
of Vertebrates 135
VIII. On the development of the spinal nerves in Elasmobranch
Fishes. With Plates 22 and 23 168
1876
IX. On the spinal nerves of Amphioxus 197
1876-78
X. A Monograph on the development of Elasmobranch
Fishes. With Plates 6-21 203
1878
XI. On the phenomena accompanying the maturation and
impregnation of the ovum 521
XII. On the structure and development of the vertebrate
ovary. With Plates 24, 25, 26 549
1879
XIII. On the existence of a Head-kidney in the Embryo Chick,
and on certain points in the development of the
Müllerian duct. By F. M. BALFOUR and A. SEDGWICK.
With Plates 27 and 28 618
XIV. On the early development of the Lacertilia, together
with some observations on the nature and relations of
the primitive Streak. With Plate 29 644
XV. On certain points in the Anatomy of Peripatus Capensis 657
XVI. On the morphology and systematic position of the
Spongida 661
1880
XVII. Notes on the development of the Araneina. With Plates
30, 31, 32 668
XVIII. On the spinal nerves of Amphioxus 696
XIX. Address to the Department of Anatomy and Physiology
of the British Association for the Advancement of
Science 698
1881
XX. On the development of the skeleton of the paired fins of
Elasmobranchii, considered in relation to its bearings
on the nature of the limbs of the Vertebrata. With
Plate 33 714
XXI. On the evolution of the Placenta, and on the possibility
of employing the characters of the Placenta in the
classification of the Mammalia 734
1882
XXII. On the structure and development of Lepidosteus. By
F. M. BALFOUR and W. N. PARKER. With Plates
34-42 738
XXIII. On the nature of the organ in Adult Teleosteans and
Ganoids which is usually regarded as the Head-kidney
or Pronephros 848
XXIV. A renewed study of the germinal layers of the Chick. By
F. M. BALFOUR and F. DEIGHTON. With Plates
43, 44, 45 854
POSTHUMOUS, 1883
XXV. The Anatomy and Development of Peripatus Capensis.
Edited by H. N. MOSELEY and A. SEDGWICK. With
Plates 46-53 871
Francis Maitland Balfour, the sixth child and third son of James Maitland
Balfour of Whittinghame, East Lothian, and Lady Blanche, daughter of the
second Marquis of Salisbury, was born at Edinburgh, during a temporary stay
of his parents there, on the 10th November, 1851. He can hardly be said to
have known his father, who died of consumption in 1856, at the early age of
thirty-six, and who spent the greater part of the last two years of his
life at Madeira, separated from the younger children who remained at home.
He fancied at one time that he had inherited his father's constitution; and
this idea seems to have spurred him on to achieve early what he had to do.
But, though there was a period soon after he went to College, during which
he seemed delicate, and the state of his health caused considerable anxiety
to his friends, he eventually became fairly robust, and that in spite of
labours which greatly taxed his strength.
The early years of his life were spent chiefly at Whittinghame under the
loving care of his mother. She made it a point to attempt to cultivate in
all her children some taste for natural science, especially for natural
history, and in this she was greatly helped by the boys' tutor, Mr J. W.
Kitto. They were encouraged to make collections and to form a museum, and
the fossils found in the gravel spread in front of the house served as the
nucleus of a geological series. Frank soon became greatly interested in
these things, and indeed they may be said to have formed the beginnings of
his scientific career. At all events there was thus awakened in him a love
for geology, which science continued to be his favorite study all through
his boyhood, and interested him to the last. He was most assiduous in
searching for fossils in the gravel and elsewhere, and so great was his
love for his collections that while as yet quite a little boy the most
delightful birthday present he could think of was a box with trays and
divisions to hold his fossils and specimens. His mother, thinking that his
fondness for fossils was a passing fancy and that he might soon regret the
purchase of the box, purposely delayed the present. But he remained
constant to his wish and in time received his box. He must at this time
have been about seven or eight years old. In the children's museum, which
has been preserved, there are specimens labelled with his childish
round-hand, such as a piece of stone with the label "marks of some shels;"
and his sister Alice, who was at that time his chief companion, remembers
discussing with him one day after the nursery dinner, when he was about
nine years old, whether it were better to be a geologist or a naturalist,
he deciding for the former on the ground that it was better to do one thing
thoroughly than to attempt many branches of science and do them
imperfectly.
Besides fossils, he collected not only butterflies, as do most boys at some
time or other, but also birds; and he with his sister Alice, being
instructed in the art of preparing and preserving skins, succeeded in
making a very considerable collection. He thus acquired before long not
only a very large but a very exact knowledge of British birds.
In the more ordinary work of the school-room he was somewhat backward. This
may have been partly due to the great difficulty he had in learning to
write, for he was not only left-handed but, in his early years, singularly
inapt in acquiring particular muscular movements, learning to dance being a
great trouble to him. Probably however the chief reason was that he failed
to find any interest in the ordinary school studies. He fancied that the
family thought him stupid, but this does not appear to have been the case.
In character he was at this time quick tempered, sometimes even violent,
and the energy which he shewed in after life even thus early manifested
itself as perseverance, which, when he was crossed, often took on the form
of obstinacy, causing at times no little trouble to his nurses and tutors.
But he was at the same time warm-hearted and affectionate; full of strong
impulses, he disliked heartily and loved much, and in his affections was
wonderfully unselfish, wholly forgetting himself in his thought for others,
and ready to do things which he disliked to please those whom he loved.
Though, as we have said, somewhat clumsy, he was nevertheless active and
courageous; in learning to ride he shewed no signs of fear, and boldly put
his pony to every jump which was practicable.
In 1861 he was sent to the Rev. C. G. Chittenden's preparatory school at
Hoddesden in Hertfordshire, and here the qualities which had been already
visible at home became still more obvious. He found difficulty not only in
writing but also in spelling, and in the ordinary school-work he took but
little interest and made but little progress.
In 1865 he was moved to Harrow and placed in the house of the Rev. F.
Rendall. Here, as at Hoddesden, he did not show any great ability in the
ordinary school studies, though as he grew older his progress became more
marked. But happily he found at Harrow an opportunity for cultivating that
love of scientific studies which was yearly growing stronger in him. Under
the care of one of the Masters, Mr G. Griffith, the boys at Harrow were
even then taught the elements of natural science. The lessons were at that
time, so to speak, extra-academical, carried on out of school hours;
nevertheless, many of the boys worked at them with diligence and even
enthusiasm, and among these Balfour became conspicuous, not only by his
zeal but by his ability. Griffith was soon able to recognize the power of
his new pupil, and thus early began to see that the pale, earnest, somewhat
clumsy-handed lad, though he gave no promise of being a scholar in the
narrower sense of the word, had in him the makings of a man of science.
Griffith chiefly confined his teaching to elementary physics and chemistry
with some little geology, but he also encouraged natural history studies
and began the formation of a museum of comparative anatomy. Balfour soon
began to be very zealous in dissecting animals, and was especially
delighted when the Rev. A. C. Eaton, the well-known entomologist, on a
visit to Harrow, initiated Griffith's pupils in the art of dissecting under
water. The dissection of a caterpillar in this way was probably an epoch in
Balfour's life. Up to that time his rough examination of such bodies had
revealed to him nothing more than what in school-boy language he spoke of
as "squash;" but when under Eaton's deft hands the intricate organs of the
larval Arthropod floated out under water and displayed themselves as a
labyrinth of threads and sheets of silvery whiteness a new world of
observation opened itself up to Balfour, and we may probably date from this
the beginning of his exact morphological knowledge.
While thus learning the art of observing, he was at the same time
developing his power of thinking. He was by nature fond of argument, and
defended with earnestness any opinions which he had been led to adopt. He
was very active in the Harrow Scientific Society, reading papers, taking
part in the discussions, and exhibiting specimens. He gained in 1867 a
prize for an essay on coal, and when, in 1868, Mr Leaf offered a prize (a
microscope) "for the best account of some locality visited by the writer
during the Easter Holidays," two essays sent in, one by Balfour, the other
by his close friend, Mr Arthur Evans, since well known for his researches
in Illyria, were found to be of such unusual merit that Prof. Huxley was
specially requested to adjudicate between them. He judged them to be of
equal merit, and a prize was given to each. The subject of Balfour's essay
was "The Geology and Natural History of East Lothian." When biological
subjects were discussed at the Scientific Society, Balfour appears to have
spoken as a most uncompromising opponent of the views of Mr Charles Darwin,
little thinking that in after life his chief work would be to develop and
illustrate the doctrine of evolution.
The years at Harrow passed quickly away, Balfour making fair, but perhaps
not more than fair, progress in the ordinary school learning. In due course
however he reached the upper sixth form, and in his last year, became a
monitor. At the same time his exact scientific knowledge was rapidly
increasing. Geology still continued to be his favorite study, and in this
he made no mean progress. During his last years at Harrow he and his
brother Gerald worked out together some views concerning the geology of
their native county. These views they ultimately embodied in a paper, which
was published in their joint names in the _Geological Magazine_ for 1872,
under the title of "Some Points in the Geology of the East Lothian Coast,"
and which was in itself a work of considerable promise. Geology however was
beginning to find a rival in natural history. Much of his holiday time was
now spent in dredging for marine animals along the coast off Dunbar. Each
specimen thus obtained was carefully determined and exact records were kept
of the various 'finds,' so that the dredgings (which were zealously
continued after he had left Harrow and gone to Cambridge) really
constituted a serious study of the fauna of this part of the coast. They
also enabled him to make a not inconsiderable collection of shells, in the
arrangement of which he was assisted by his sister Evelyn, of crustacea and
of other animals.
Both to the masters and to his schoolfellows he became known as a boy of
great force of character. Among the latter his scrupulous and unwavering
conscientiousness made him less popular perhaps than might have been
expected from his bright kindly manner and his unselfish warmheartedness.
In the incidents of school life a too strict conscience is often an
inconvenience, and the sternness and energy with which Balfour denounced
acts of meanness and falsehood were thought by some to be unnecessarily
great. He thus came to be feared rather than liked by many, and
comparatively few grew to be sufficiently intimate with him to appreciate
the warmth of his affections and the charm of his playful moments.
At the Easter of 1870 he passed the entrance examination at Trinity
College, Cambridge, and entered into residence in the following October.
His college tutor was Mr J. Prior, but he was from the first assisted and
guided in his studies by his friend, Mr Marlborough Pryor, an old Harrow
boy, who in the same October had been, on account of his distinction in
Natural Science, elected a Fellow of the College, in accordance with
certain new regulations which then came into action for the first time, and
which provided that every three years one of the College Fellowships should
be awarded for excellence in some branch or branches of Natural Science, as
distinguished from mathematics, pure or mixed. During the whole of that
year and part of the next Mr Marlborough Pryor remained in residence, and
his influence in wisely directing Balfour's studies had a most beneficial
effect on the latter's progress.
During his first term Balfour was occupied in preparation for the Previous
Examination; and this he successfully passed at Christmas. After that he
devoted himself entirely to Natural Science, attending lectures on several
branches. During the Lent term he was a very diligent hearer of the
lectures on Physiology which I was then giving as Trinity Prælector, having
been appointed to that post in the same October that Balfour came into
residence. At this time he was not very strong, and I remember very well
noticing among my scanty audience, a pale retiring student, whose mind
seemed at times divided between a desire to hear the lecture and a feeling
that his frequent coughing was growing an annoyance to myself and the
class. This delicate-looking student, I soon learnt, was named Balfour, and
when the Rev. Coutts Trotter, Mr Pryor and myself came to examine the
candidates for the Natural Science Scholarships which were awarded at
Easter, we had no difficulty in giving the first place to him. In point of
knowledge, and especially in the thoughtfulness and exactitude displayed in
his papers and work, he was very clearly ahead of his competitors.
During the succeeding Easter term and the following winter he appears to
have studied physics, chemistry, geology and comparative anatomy, both
under Mr Marlborough Pryor and by means of lectures. He also continued to
attend my lectures, but though I gradually got to know him more and more we
did not become intimate until the Lent term of 1872. He had been very much
interested in some lectures on embryology which I had given, and, since
Marlborough Pryor had left or was about to leave Cambridge, he soon began
to consult me a good deal about his studies. He commenced practical
histological and embryological work under me, and I remember very vividly
that one day when we were making a little excursion in search of nests and
eggs of the stickleback in order that he might study the embryology of
fishes, he definitely asked my opinion as to whether he might take up a
scientific career with a fair chance of success. I had by this time formed
a very high opinion of his abilities, and learning then for the first time
that he had an income independent of his own exertions, my answer was very
decidedly a positive one. Soon after, feeling more and more impressed with
his power and increasingly satisfied both with his progress in biological
studies and his sound general knowledge of other sciences, anxious also, it
may be, at the same time that as much original inquiry as possible should
be carried on at Cambridge in my department, I either suggested to him or
acquiesced in his own suggestion that he should at once set to work on some
distinct research; and as far as I remember the task which I first proposed
to him was an investigation of the layers of the blastoderm in the chick.
It must have been about the same time that I proposed to him to join me in
preparing for publication a small work on Embryology, the materials for
this I had ready to hand in a rough form as lectures which I had previously
given. To this proposal he enthusiastically assented, and while the lighter
task of writing what was to be written fell to me, he undertook to work
over as far as was possible the many undetermined points and unsatisfactory
statements across which we were continually coming.
During his two years at College his health had improved; though still
hardly robust and always in danger of overworking himself, he obviously
grew stronger. He rejoiced exceedingly in his work, never tiring of it, and
was also making his worth felt among his fellow students, and especially
perhaps among those of his own college whose studies did not lie in the
same direction as his own. At this time he must have been altogether happy,
but a sorrow now came upon him. His mother, to whom he was passionately
attached, and to whose judicious care in his early days not only the right
development of his strong character but even his scientific leanings were
due, had for some time past been failing in health, though her condition
caused no immediate alarm. In May 1872, however, she died quite suddenly
from unsuspected heart disease. Her loss was a great blow to him, and for
some time afterward I feared his health would give way; but he bore his
grief quietly and manfully and threw himself with even increased vigour
into his work.
During the academic session of 1872-3, he continued steadily at work at his
investigations, and soon began to make rapid progress. At the beginning he
had complained to me about what he considered his natural clumsiness, and
expressed a fear that he should never be able to make satisfactory
microscopic sections; as to his being able to make drawings of his
dissections and microscopical preparations, he looked upon that at first as
wholly impossible. I need hardly say that in time he acquired great skill
in the details of microscopical _technique_, and that his drawings, if
wanting in so-called artistic finish, were always singularly true and
instructive. While thus struggling with the details which I could teach
him, he soon began to manifest qualities which no teacher could give him. I
remember calling his attention to Dursy's paper on the Primitive Streak,
and suggesting that he should work the matter over, since if such a
structure really existed, it must, most probably, have great morphological
significance. I am free to confess that I myself rather doubted the matter,
and a weaker student might have been influenced by my preconceptions.
Balfour, however, thus early had the power of seeing what existed and of
refusing to see what did not exist. He was soon able to convince me that
Dursy's streak was a reality, and the complete working out of its
significance occupied his thoughts to the end of his days.
The results of these early studies were made known in three papers which
appeared in the _Quarterly Journal of Microscopical Science_ for July 1873,
and will be found in the beginning of this volume. The summer and autumn of
that year were spent partly in a visit to Finland, in company with his
friend and old school-fellow Mr Arthur Evans, and partly in formal
preparation for the approaching Tripos examination. Into this preparation
Balfour threw himself with characteristic energy, and fully justified my
having encouraged his spending so much of the preceding time in original
research, not only by the rapidity with which he accumulated the stock of
knowledge of various kinds necessary for the examination but also by the
manner in which he acquitted himself at the trial itself. At that time the
position of the candidates in the Natural Sciences Tripos was determined by
the total number of marks, and Balfour was placed second, the first place
being gained by H. Newell Martin of Christ's College, now Professor at
Baltimore, U.S.A. In the examination, in which I took part, Balfour did not
write much, and he had not yet learnt the art of putting his statements in
the best possible form; he won his position chiefly by the firm thought and
clear insight which was present in almost all his answers.
The examination was over in the early days of Dec. 1873 and Balfour was now
free to devote himself wholly to his original work. Happily, the University
had not long before secured the use of two of the tables at the then
recently founded Stazione Zoologica at Naples. And upon the nomination of
the University, Balfour, about Christmas, started for Naples in company
with his friend Mr A. G. Dew-Smith, also of Trinity College. The latter was
about to carry on some physiological observations; Balfour had set himself
to work out as completely as he could the embryology of Elasmobranch
fishes, about which little was at that time known, but which, from the
striking characters of the adult animals could not help proving of interest
and importance.
From his arrival there at Christmas 1873 until he left in June 1874, he
worked assiduously, and with such success, that as the result of the
half-year's work he had made a whole series of observations of the greatest
importance. Of these perhaps the most striking were those on the
development of the urogenital organs, on the neurenteric canal, on the
development of the spinal nerves, on the formation of the layers and on the
phenomena of segmentation, including a history of the behaviour of nuclei
in cell division. He returned home laden with facts and views both novel
and destined to influence largely the progress of embryology.
In August of the same year he attended the meeting of the British
Association for the Advancement of Science at Belfast; and the account he
then gave of his researches formed one of the most important incidents at
the Biological Section on that occasion.
In the September of that year the triennial fellowship for Natural Science
was to be awarded at Trinity College, and Balfour naturally was a
candidate. The election was, according to the regulations, to be determined
partly by the result of an examination in various branches of science, and
partly by such evidence of ability and promise as might be afforded by
original work, published or in manuscript. He spent the remainder of the
autumn in preparation for this examination. But when the examination was
concluded it was found that in his written answers he had not been very
successful; he had not even acquitted himself so well as in the Tripos of
the year before, and had the election been determined by the results of the
examination alone, the examiners would have been led to choose the
gentleman who was Balfour's only competitor. The original work however
which Balfour sent in, including a preliminary account of the discoveries
made at Naples, was obviously of so high a merit and was spoken of in such
enthusiastic terms by the External Referee Prof. Huxley, that the examiners
did not hesitate for a moment to neglect altogether the formal written
answers (and indeed the papers of questions were only introduced as a
safeguard, or as a resource in case evidence of original power should be
wanted) and unanimously recommended him for election. Accordingly he was
elected Fellow in the early days of October.
Almost immediately after, the little book on Embryology appeared, on which
he and I had been at work, he doing his share even while his hands and mind
were full of the Elasmobranch inquiry. The title-page was kept back some
little time in order that his name might appear on it with the addition of
Fellow of Trinity, a title of which he was then, and indeed always
continued to be, proud. He also published in the October number of the
_Quarterly Journal of Microscopical Science_ a preliminary account of his
Elasmobranch researches.
He and his friends thought that after these almost incessant labours, and
the excitement necessarily contingent upon the fellowship election, he
needed rest and change. Accordingly on the 17th of October he started with
his friend Marlborough Pryor on a voyage to the west coast of South
America. They travelled thither by the Isthmus of Panama, visited Peru and
Chili, and returned home along the usual route by the Horn; reaching
England some time in Feb. 1875.
Refreshed by this holiday, he now felt anxious to complete as far as
possible his Elasmobranch work, and very soon after his return home, in
fact in March, made his way again to Naples, where he remained till the hot
weather set in in May. On his return to Cambridge, he still continued
working on the Elasmobranchii, receiving material partly from Naples,
partly from the Brighton Aquarium, the then director of which, Mr Henry
Lee, spared no pains to provide him both with embryo and adult fishes.
While at Naples, he communicated to the Philosophical Society at Cambridge
a remarkable paper on "The Early Stages of Vertebrates," which was
published in full in the _Quarterly Journal of Microscopical Science_,
July, 1875; he also sent me a paper on "The Development of the Spinal
Nerves", which I communicated to the Royal Society, and which was
subsequently published in the _Philosophical Transactions_ of 1876. He
further wrote in the course of the summer and published in the _Journal of
Anatomy and Physiology_ in October, 1875, a detailed account of his
"Observations and Views on the Development of the Urogenital Organs."
Some time in August of the same year he started in company with Mr Arthur
Evans and Mr J. F. Bullar for a second trip to Finland, the travellers on
this occasion making their way into regions very seldom visited, and having
to subsist largely on the preserved provisions which they carried with
them, and on the produce of their rods and guns. From a rough diary which
Balfour kept during this trip it would appear that while enjoying heartily
the fun of the rough travelling, he occupied himself continually with
observations on the geology and physical phenomena of the country, as well
as on the manners, antiquities, and even language of the people. It was one
of his characteristic traits, a mark of the truly scientific bent of his
mind, of his having, as Dohrn soon after Balfour's first arrival at Naples
said, 'a real scientific head,' that every thing around him wherever he
was, incited him to careful exact observation, and stimulated him to
thought.
In the early part of the Long Vacation of the same year he had made his
first essay in lecturing, having given a short course on Embryology in a
room at the New Museums, which I then occupied as a laboratory. Though he
afterwards learnt to lecture with great clearness he was not by nature a
fluent speaker, and on this occasion he was exceedingly nervous. But those
who listened to him soon forgot these small defects as they began to
perceive the knowledge and power which lay in their new teacher.
Encouraged by the result of this experiment, he threw himself, in spite of
the heavy work which the Elasmobranch investigation was entailing, with
great zeal into an arrangement which Prof. Newton, Mr J. W. Clark and
myself had in course of the summer brought about, that he and Mr A. Milnes
Marshall, since Professor at Owens College, Manchester, should between them
give a course on Animal Morphology, with practical instruction, Prof.
Newton giving up a room in the New Museums for the purpose.
In the following October (1875) upon Balfour's return from Finland, these
lectures were accordingly begun and carried on by the two lecturers during
the Michaelmas and Lent Terms. The number of students attending this first
course, conducted on a novel plan, was, as might be expected, small, but
the Lent Term did not come to an end before an enthusiasm for morphological
studies had been kindled in the members of the class.
The ensuing Easter term (1876) was spent by Balfour at Naples, in order
that he might carry on towards completion his Elasmobranch work. He had by
this time determined to write as complete a monograph as he could of the
development of these fishes, proposing to publish it in instalments in the
_Journal of Anatomy and Physiology_, and subsequently to gather together
the several papers into one volume. The first of these papers, dealing with
the ovum, appeared in Jan. 1876; most of the numbers of the Journal during
that and the succeeding year contained further portions; but the complete
monograph did not leave the publisher's hands until 1878.
He returned to England with his pupil and friend Mr J. F. Bullar some time
in the summer; on their way home they passed through Switzerland, and it
was during the few days which he then spent in sight of the snow-clad hills
that the beginnings of a desire for that Alpine climbing, which was
destined to be so disastrous, seem to have been kindled in him.
In October, 1876, he resumed the lectures on Morphology, taking the whole
course himself, his colleague, Mr Marshall, having meanwhile left
Cambridge. Indeed, from this time onward, he may be said to have made these
lectures, in a certain sense, the chief business of his life. He lectured
all three terms, devoting the Michaelmas and Lent terms to a systematic
course of Animal Morphology, and the Easter term to a more elementary
course of Embryology. These lectures were given under the auspices of Prof.
Newton; but Balfour's position was before long confirmed by his being made
a Lecturer of Trinity College, the lectures which he gave at the New
Museums, and which were open to all students of the University, being
accepted in a liberal spirit by the College as equivalent to College
Lectures. He very soon found it desirable to divide the morphological
course into an elementary and an advanced course, and to increase the
number of his lectures from three to four a week. Each lecture was followed
by practical work, the students dissecting and examining microscopically,
an animal or some animals chosen as types to illustrate the subject-matter
of the lecture; and although Balfour had the assistance at first of one[2],
and ultimately of several demonstrators, he himself put his hand to the
plough, and after the lecture always spent some time in the laboratory
among his pupils. Had Balfour been only an ordinary man, the zeal and
energy which he threw into his lectures, and into the supervision of the
practical work, added to the almost brotherly interest which he took in the
individual development of every one of the pupils who shewed any love
whatever for the subject, would have made him a most successful teacher.
But his talents and powers were such as could not be hid even from
beginners. His extensive and exact knowledge, the clearness with which in
spite of, or shall I not rather say, by help of a certain want of fluency,
he explained difficult and abstruse matters, the trenchant way in which he
lay bare specious fallacies, and the presence in almost his every word of
that power which belongs only to the man who has thought out for himself
everything which he says, these things aroused and indeed could hardly fail
to arouse in his hearers feelings which, except in the case of the very
dullest, grew to be those of enthusiasm. His class, at first slowly, but
afterwards more rapidly, increased in numbers, and, what is of more
importance, grew in quality. The room allotted to him soon became far too
small and steps were taken to provide for him, for myself, whose wants were
also urgent, and for the biological studies generally, adequate
accommodation; but it was not until Oct. 1877 that we were able to take
possession of the new quarters.
Footnote 2: His first Demonstrator up to Christmas 1877, was
Mr J. F. Bullar. In Jan. 1878, Mr Adam Sedgwick took the post
of Senior Demonstrator, and held it until Balfour's death.
Even this new accommodation soon became insufficient, and in the spring of
1882 a new morphological laboratory was commenced in accordance with plans
suggested by himself. He was to have occupied them in the October term,
1883, but did not live to see them finished.
As might have been expected from his own career, he regarded the mere
teaching of what is known as a very small part of his duties as Lecturer;
and as soon as any of his pupils became sufficiently advanced, he urged or
rather led them to undertake original investigations; and he had the
satisfaction before his death of seeing the researches of his pupils (such
as those by Messrs. Bullar, Sedgwick, Mitzikuri, Haddon, Scott, Osborne,
Caldwell, Heape, Weldon, Parker, Deighton and others) carried to a
successful end. In each of these inquiries he himself took part, sometimes
a large part, generally suggesting the problem to be solved, indicating the
methods, and keeping a close watch over the whole progress of the study.
Hence in many cases the published account bears his name as well as that of
the pupil.
In the year 1878 his _Monograph on Elasmobranch Fishes_ was published as a
complete volume, and in the same year he received the honour of being
elected a Fellow of the Royal Society, a distinction which now-a-days does
not often fall to one so young. No sooner was the Monograph completed than
in spite of the labours which his lectures entailed, he set himself to the
great task of writing a complete treatise on _Comparative Embryology_. This
not only laid upon him the heavy burden of gathering together the
observations of others, enormous in number and continually increasing,
scattered through many journals and books, and recorded in many different
languages, as well as of putting them in orderly array, and of winnowing
out the grain from the chaff (though his critical spirit found some relief
in the latter task), but also caused him much labour, inasmuch as at almost
every turn new problems suggested themselves, and demanded inquiry before
he could bring his mind to writing about them. This desire to see his way
straight before him, pursued him from page to page, and while it has
resulted in giving the book an almost priceless value, made the writing of
it a work of vast labour. Many of the ideas thus originated served as the
bases of inquiries worked out by himself or his pupils, and published in
the form of separate papers, but still more perhaps never appeared either
in the book or elsewhere and were carried with him undeveloped and
unrecorded to the grave.
The preparation of this work occupied the best part of his time for the
next three years, the first volume appearing in 1880, the second in 1881.
In the autumn of 1880, he attended the Meeting at Swansea of the British
Association for the Advancement of Science, having been appointed
Vice-President of the Biological Section with charge of the Department of
Anatomy and Physiology. At the Meetings of the Association, especially of
late years, much, perhaps too much, is expected in the direction of
explaining the new results of science in a manner interesting to the
unlearned. Popular expositions were never very congenial to Balfour, his
mind was too much occupied with the anxiety of problems yet to be solved;
he was therefore not wholly at his ease, in his position on this occasion.
Yet his introductory address, though not of a nature to interest a large
mixed audience, was a luminous, brief exposition of the modern development
and aims of embryological investigation.
During these years of travail with the _Comparative Embryology_ the amount
of work which he got through was a marvel to his friends, for besides his
lectures, and the researches, and the writing of the book, new labours were
demanded of him by the University for which he was already doing so much.
Men at Cambridge, and indeed elsewhere as well, soon began to find out that
the same clear insight which was solving biological problems could be used
to settle knotty questions of policy and business. Moreover he united in a
remarkable manner, the power of boldly and firmly asserting and maintaining
his own views, with a frank courteousness which went far to disarm
opponents. Accordingly he found himself before long a member of various
Syndicates, and indeed a very great deal of his time was thus occupied,
especially with the Museums and Library Syndicates, in both of which he
took the liveliest interest. Besides these University duties his time and
energy were also at the service of his College. In the preparation of the
New Statutes, with which about this time the College was much occupied, the
Junior Fellows of the College took a conspicuous share, and among these
Junior Fellows Balfour was perhaps the most active; indeed he was their
leader, and he threw himself into the investigation of the bearings and
probable results of this and that proposed new statute with as much zeal as
if he were attacking some morphological problem.
While he was in the midst of these various labours, his friends often
feared for his strength, for though gradually improving in health after his
first year at Cambridge, he was not robust, and from time to time he seemed
on the point of breaking down. Still, hard as he was working, he was in
reality wisely careful of himself, and as he grew older, paid more and more
attention to his health, daily taking exercise in the form either of
bicycle rides or of lawn-tennis. Moreover he continued to spend some part
of his vacations in travel. Combining business with pleasure, he made
frequent visits to Germany and France, and especially to Naples. The
Christmas of 1876-7 he spent in Greece, that of 1878-9 at Ragusa, where his
old school-fellow and friend Mr Arthur Evans was at that time residing, and
the appointment of his friend Kleinenberg to a Professorship at Messina led
to a journey there. Early in the long vacation of 1880, he went with his
sister, Mrs H. Sidgwick, and her husband to Switzerland, and was joined
there for a short time by his friend and pupil Adam Sedgwick. During this
visit he took his first lessons in Alpine climbing, making several
excursions, some of them difficult and dangerous; and the love of
mountaineering laid so firm a hold upon him, that he returned to
Switzerland later on in the autumn of the same year, in company with his
brother Gerald, and spent some weeks near Zermatt in systematic climbing,
ascending, among other mountains, the Matterhorn and the Weisshorn. In the
following summer, 1881, he and his brother Gerald again visited the Alps,
dividing their time between the Chamonix district and the Bernese Oberland.
On this occasion some of the excursions which they made were of extreme
difficulty, and such as needed not only great presence of mind and bodily
endurance, but also skilful and ready use of the limbs. As a climber indeed
Balfour soon shewed himself fearless, indefatigable, and expert in all
necessary movements as well as full of resources and expedients in the face
of difficulties, so much so that he almost at once took rank among the
foremost of distinguished mountaineers. In spite of his apparent clumsiness
in some matters, he had even as a lad proved himself to be a bold and
surefooted climber. Moreover he had been perhaps in a measure prepared for
the difficulties of Alpine climbing by his experience in deer-stalking.
This sport he had keenly and successfully pursued for many years at his
brother's place in Rosshire. When however about the year 1877, the question
of physiological experiments on animals became largely discussed in public,
he felt that to continue the pursuit of this or any other sport involving,
for the sake of mere pleasure, the pain and death of animals, was
inconsistent with the position which he had warmly taken up, as an advocate
of the right to experiment on animals; and he accordingly from that time
onward wholly gave it up.
His fame as an investigator and teacher, and as a man of brilliant and
powerful parts, was now being widely spread. Pupils came to him, not only
from various parts of England, but from America, Australia and Japan. At
the York Meeting of the British Association for the Advancement of Science,
in August, 1881, he was chosen as one of the General Secretaries. In April,
1881, the honorary degree of LL.D. was conferred upon him by the University
of Glasgow, and in November of the same year the Royal Society gave him one
of the Royal Medals in recognition of his embryological discoveries, and at
the same time placed him on its Council.
At Cambridge he was chosen, in the autumn of 1880, President of the
Philosophical Society, and in the December of that year a brilliant company
were gathered together at the Annual Dinner to do honour to their new young
President. Otherwise nothing as yet had been done for him in his own
University in the way of recognition of his abilities and services; and he
still remained a Lecturer of Trinity College, giving lectures in a
University building. An effort had been made by some of his friends to urge
the University to take some step in this direction; but it was thought at
that time impossible to do anything. In 1881 a great loss fell upon the
sister University of Oxford in the death of Prof. George Rolleston; and
soon after very vigorous efforts were made to induce Balfour to become a
candidate for the vacant chair. The prospect was in many ways a tempting
one, and Balfour seeing no very clear way in the future for him at his own
University, was at times inclined to offer himself, but eventually he
decided to remain at Cambridge. Hardly had this temptation if we may so
call it been overcome when a still greater one presented itself. Through
the lamented death of Sir Wyville Thomson in the winter of 1881-2, the
chair of Natural History at Edinburgh, perhaps the richest and most
conspicuous biological chair in the United Kingdom, became vacant. The post
was in many ways one which Balfour would have liked to hold. The teaching
duties were it is true laborious, but they had in the past been compressed
into a short time, occupying only the summer session and leaving the rest
of the year free, and it seemed probable that this arrangement might be
continued with him. The large emolument would also have been grateful to
him inasmuch as he would have felt able to devote the whole of it to
scientific ends; and the nearness to Whittinghame, his native place and
brother's home, added to the attractions; but what tempted him most was the
position which it would have given him, and the opportunities it would have
afforded, with the rich marine Fauna of the north-eastern coast close at
hand, to develop a large school of Animal Morphology. The existing
Professors at Edinburgh were most desirous that he should join them, and
made every effort to induce him to come. On the part of the Crown, in whose
hands the appointment lay, not only were distinct offers made to him, but
he was repeatedly pressed to accept the post. Nor was it until after a
considerable struggle that he finally refused, his love for his own
University in the end overcoming the many inducements to leave; he elected
to stay where he was, trusting to the future opening up for him some
suitable position. In this decision he was undoubtedly influenced by the
consideration that Cambridge, besides being the centre of his old
friendships, had become as it were a second home for his own family. By the
appointment of Lord Rayleigh to the chair of Experimental Physics his
sister Lady Rayleigh had become a resident, his sister Mrs Sidgwick had
lived there now for some years, and his brother Gerald generally spent the
summer there; their presence made Cambridge doubly dear to him.
At the close of the Michaelmas term, with feelings of relief at having
completed his _Comparative Embryology_, the preparation of the second
volume of which had led to almost incessant labour during the preceding
year, he started to spend the Christmas vacation with his friend
Kleinenberg at Messina. Stopping at Naples on his way thither he found his
pupil Caldwell, who had been sent to occupy the University table at the
Stazione Zoologica, lying ill at Capri, with what proved to be typhoid
fever. The patient was alone, without any friend to tend him, and his
mother who had been sent for had not yet arrived. Accordingly Balfour (with
the kindness all forgetful of himself which was his mark all his life
through) stayed on his journey to nurse the sick man until the mother came.
He then went on to Messina, and there seemed to be in good health, amusing
himself with the ascent of Etna. Yet in January, soon after his return
home, he complained of being unwell, and in due time distinct symptoms of
typhoid fever made their appearance. The attack at first promised to be
severe, but happily the crisis was soon safely passed and the convalescence
was satisfactory.
While yet on his sick bed, a last attempt was made to induce him to accept
the Edinburgh offer, and for the last time he refused. These repeated
offers, and the fact that the dangers of his grave illness had led the
University vividly to realize how much they would lose if Balfour were
taken away from them, encouraged his friends to make a renewed effort to
gain for him some adequate position in the University. This time the
attempt was successful, and the authorities took a step, unusual but
approved of by the whole body of resident members of the University; they
instituted a new Professorship of Animal Morphology, to be held by Balfour
during his life or as long as he should desire, but to terminate at his
death or resignation unless it should be otherwise desirable. Accordingly
in May, 1882, he was admitted into the Professoriate as Professor of Animal
Morphology.
During his illness his lectures had been carried on by his Demonstrator, Mr
Adam Sedgwick, who continued to take his place during the remainder of that
Lent Term and during the ensuing Easter Term. The spring Balfour spent
partly in the Channel Islands with his sister Alice, partly in London with
his eldest brother, but in the course of the Easter Term returned to
Cambridge and resumed his work though not his lectures. His recovery to
health was steady and satisfactory, the only drawback being a swelling over
the shin-bone of one leg, due to a blow on the rocks at Sark; otherwise he
was rapidly becoming strong. He himself felt convinced that a visit to the
Alps, with some mountaineering of not too difficult a kind, would complete
his restoration to health. In this view many of his friends coincided; for
the experience of former years had shewn them what a wonderfully beneficial
effect the Alpine air and exercise had upon his health. He used to go away
pale, thin and haggard, to return bronzed, clear, firm and almost stout;
nor was there anything in his condition which seemed to forbid his
climbing, provided that he was cautious at the outset. Accordingly, early
in June he left Cambridge for Switzerland, having long ago, during his
illness in fact, engaged his old guide, Johann Petrus, whom he had first
met in 1880, and who had always accompanied him in his expeditions since.
His first walking was in the Chamonix district; and here he very soon found
his strength and elasticity come back to him. Crossing over from Montanvert
to Courmayeur, by the Col du Géant, he was attracted by the peak called the
Aiguille Blanche de Peuteret, a virgin peak, the ascent of which had been
before attempted but not accomplished. Consulting with Petrus he determined
to try it, feeling that the fortnight, which by this time he had spent in
climbing, had brought back to him his old vigour, and that his illness was
already a thing of the past.
There is no reason to believe that he regarded the expedition as one of
unusual peril; and an incident which at the time of his death was thought
by some to indicate this was in reality nothing more than a proof of his
kindly foresight. The guide Petrus was burdened by a debt on his land
amounting to about £150. In the previous year Balfour and his brother had
come to know of this debt; and, seeing that no Alpine ascent is free from
danger, that on any expedition some accident might carry them off, had
conceived the idea of making some provision for Petrus' family in case he
might meet with sudden death in their service. This suggestion of the
previous year Balfour carried out on this occasion, and sent home to his
brother Gerald a cheque of £150 for this purpose. But the cheque was sent
from Montanvert before he had even conceived the idea of ascending the
Aiguille Blanche. It was not a provision for any specially dangerous
ascent, and must be regarded as a measure prompted not by a sense of coming
peril but rather by the donor's generous care for his servant.
On Tuesday afternoon, July 18, he and Petrus, with a porter to carry
provisions and firing to their sleeping-place on the rocks, set out from
Courmayeur, the porter returning the same night. They expected to get back
to Courmayeur some time on the Thursday, but the day passed without their
appearing. This did not cause any great anxiety because it was supposed
that they might have found it more convenient to pass over to the Chamonix
side than to return to Courmayeur. When on Friday however telegrams
dispatched to Chamonix and Montanvert brought answers that nothing had been
seen of them, it became evident that some accident had happened, and an
exploring party set out for the hills. It was not until early on the Sunday
morning that this search party found the bodies, both partly covered with
snow, lying on the Glacier de Fresney, below the impassable icefall which
separates the upper basin of the glacier from the lower portion, and at the
foot of a _couloir_ which descends by the side of the icefall. Their tracks
were visible on the snow at the top of the _couloir_. Balfour's neck was
broken, and his skull fractured in three places; Petrus' body was also
fractured in many places. The exact manner of their death will never be
known, but there can be no doubt that, in Balfour's case at all events, it
was instantaneous, and those competent to form a judgment are of opinion
that they were killed by a sudden fall through a comparatively small
height, slipping on the rocks as they were descending by the side of the
ice-fall, and not precipitated from the top of the _couloir_. There is
moreover indirect evidence which renders it probable that in the fatal fall
Petrus slipped first and carried Balfour with him. Whether they had reached
the summit of the Aiguille and were returning home after a successful
ascent or whether they were making their way back disheartened and wearied
with failure, is not and perhaps never will be known. Since the provisions
at the sleeping-place were untouched, the deaths probably took place on
Wednesday the 19th. The bringing down the bodies proved to be a task of
extreme difficulty, and it was not till Wednesday the 26th that the remains
reached Courmayeur, where M. Bertolini, the master of the hotel, and indeed
everyone, not least the officers of a small body of Italian troops
stationed there, shewed the greatest kindness and sympathy to Balfour's
brothers, Gerald and Eustace, who hastened to the spot as soon as the news
of the terrible disaster was telegraphed home. Mr Walter Leaf also and Mr
Conway, friends of Balfour, the former a very old one, who had made their
way to Courmayeur from other parts of Switzerland as soon as they heard of
the accident, rendered great assistance. The body was embalmed, brought to
England, and buried at Whittinghame on Saturday, Aug. 5, the Fellows of
Trinity College holding a service in the College Chapel at the same time.
In person he was tall, being fully six feet in height, well built though
somewhat spare. A broad forehead overhanging deeply set dark brown eyes
whose light shining from beneath strongly marked eye-brows told all the
changes of his moods, slightly prominent cheek-bones, a pale skin, at times
inclined to be even sallow, dark brown hair, allowed to grow on the face
only as a small moustache, and slight whiskers, made up a countenance which
bespoke at once strength of character and delicacy of constitution. It was
an open countenance, hiding nothing, giving sign at once, both when his
body was weary or weak, and when his mind was gladdened, angered or
annoyed.
The record of some of his thoughts and work, all that he had given to the
world will be found in the following pages. But who can tell the ideas
which had passed into his quick brain, but which as yet were known only to
himself, of which he had given no sign up to that sad day on which he made
the fatal climb? And who can say whither he might not have reached had he
lived, and his bright young life ripened as years went on? This is not the
place to attempt any judgment of his work: that may be left to other times,
and to other hands; but it may be fitting to place here on record a letter
which shews how much the greatest naturalist of this age appreciated his
younger brother. Among Balfour's papers was found a letter from Charles
Darwin, acknowledging the receipt of Vol. II. of the _Comparative
Embryology_ in the following words:
"_July 6, 1881._
DOWN, BECKENHAM, KENT.
MY DEAR BALFOUR,
I thank you heartily for the present of your grand book, and
I congratulate you on its completion. Although I read almost
all of Vol. I, I do not feel that I am worthy of your
present, unless indeed the fullest conviction that it is a
memorable work makes me worthy to receive it.
* * * * *
Once again accept my thanks, for I am proud to receive a
book from you, who, I know, will some day be the chief of
the English Biologists.
Believe me,
Yours sincerely,
CHARLES DARWIN."
The loss of him was a manifold loss. He is mourned, and will long be
mourned, for many reasons. Some miss only the brilliant investigator;
others feel that their powerful and sympathetic teacher is gone; some look
back on his memory and grieve for the charming companion whose kindly
courtesy and bright wit made the hours fly swiftly and pleasantly along;
and to yet others is left an aching void when they remember that they can
never again lean on the friend whose judgment seemed never to fail and
whose warm-hearted affection was a constant help. And to some he was all of
these. At the news of his death the same lines came to the lips of all of
us, so fittingly did Milton's words seem to speak our loss and grief--
"For Lycidas is dead, dead ere his prime,
Young Lycidas, and hath not left his peer."
M. FOSTER.
I. ON SOME POINTS IN THE GEOLOGY OF THE EAST LOTHIAN COAST[3].
Footnote 3: From the _Geological Magazine_, Vol. IX. No. 4.
April, 1872.
By G. W. and F. M. BALFOUR, Trinity College, Cambridge.
The interesting relation between the Porphyrite of Whitberry Point, at the
mouth of the Tyne, near Dunbar, and the adjacent sedimentary rocks, was
first noticed, we believe, by Professor Geikie, who speaks of it in the
_Memoirs of the Geological Survey of East Lothian_, pages 40 and 31, and
again in the new edition of Jukes's _Geology_, p. 269. The volcanic mass
which forms the point consists of a dark felspathic base with numerous
crystals of augite: it is circular in form, and is exposed for two-thirds
of its circumference in a vertical precipice facing the sea, about twenty
feet in height.
The rock is traversed by numerous joints running both in a horizontal and
in a vertical direction. The latter are by far the most conspicuous, and
give the face of the cliff, when seen from a distance, a well-marked
columnar appearance, though the columns themselves are not very distinct or
regular. They are quadrangular in form, and are evidently produced by the
intersection at right-angles of the two series of vertical joints.
It is clear that the face of the precipice has been gradually receding in
proportion as it yielded to the action of the waves; and that at a former
period the volcanic rock extended considerably further than at present over
the beds which are seen to dip beneath it. These latter consist of hard
fine-grained calcareous sandstones belonging to the Lower Carboniferous
formation. Their colour varies from red to white, and their prevailing dip
is in a N.W. direction, with an average inclination of 12-20°. If the
volcanic mass is a true intrusive rock, we should naturally expect the
strata which surround it to dip _away_ in all directions, the amount of
their inclination diminishing in proportion to their distance from it. We
find, however, that the case is precisely the reverse: as the beds approach
the base of the cliff, they dip _towards_ it from every side at perpetually
increasing angles, until at the point of junction the inclination amounts
in places to as much as 55 degrees. The exact amount of dip in the various
positions will be seen on referring to the accompanying map.
[Illustration: FIG. 1. MAP OF STRATA AT WHITBERRY POINT. Scale, 6 in. to
the mile.
_A._ Lava sheet. _B._ Sandstone Beds, dipping from every side towards the
lava. _CC._ Line of Section along which Fig. 2 is supposed to be drawn.]
We conceive that the phenomenon is to be explained by supposing the orifice
through which the lava rose and overflowed the surface of the sedimentary
strata to have been very much smaller in area than the extent of igneous
rock at present visible; and that the pressure of the erupted mass on the
soft beds beneath, aided perhaps by the abstraction of matter from below,
caused them to incline towards the central point at a gradually increasing
angle. The diagram, fig. 2, will serve further to illustrate this
hypothesis. _A_ is the neck or orifice by which the melted matter is
supposed to ascend. _C_ shews the sheet of lava after it has overspread the
surface of the sandstone beds _B_, so as to cause them to assume their
present inclination. The dotted lines represent the hypothetical extension
of the igneous mass and sandstones previous to the denudation which they
have suffered from the action of the waves.
Professor Geikie, in his admirable treatise on the Geology of the
county[4], adopts a view on this subject which is somewhat different from
that which is suggested in this paper. He considers that the whole mass is
an intrusive neck of rock with perpendicular sides; and that it once filled
up an orifice through the surrounding sedimentary strata, of which it is
now the only remnant.
Footnote 4: _Memoirs of Geological Survey of Scotland_, sheet
33, pp. 40, 41.
[Illustration: FIG. 2. VERTICAL SECTION THROUGH CC. DIAGRAM (FIG. 1).
_A._ Orifice by which the lava ascended. _B._ Sandstone Beds. _B´._
Hypothetical extension of ditto. _C._ Sheet of lava spread over the
sandstones _B_. _C´_. Hypothetical extension of ditto.]
He admits that the inclination of the sandstone beds towards the igneous
mass in the centre is a phenomenon that is somewhat difficult to explain,
and suggests that a subsequent contraction of the column may have tended to
produce such a result. To use his own words: "In the case of a solid column
of felstone or basalt, the contraction of the melted mass on cooling may
have had some effect in dragging down the sides of the orifice[5]."
Footnote 5: Note on p. 41 of _Mem. Geol. Survey of East
Lothian_.
But, apart from other objections, it is scarcely conceivable that this
result should have been produced by the contraction of the column.
In his recent edition of Jukes's _Manual of Geology_ (p. 269), in which he
also refers to this instance, he states that in other cases of "necks" it
is found to be an almost invariable rule, "that strata are bent down so as
to dip into the neck all round its margin." We are not aware to what other
instances Prof. Geikie may allude; but on referring to his _Memoir on the
Geology of East Lothian_, we find that he states in the cases of 'North
Berwick Law' and 'Traprain' (which he compares with the igneous mass at
Whitberry Point), that the beds at the base of these two necks, where
exposed, dip _away_ from them, and that at a high angle.
In support of the hypothesis which we have put forward, the following
arguments may be urged:
(1) That in one place at least the sedimentary strata are seen to be
actually dipping beneath the superincumbent basalt; and that the impression
produced by the general relation of the two rocks is, that they do so
everywhere.
(2) Since the columns into which the lava is split are vertical, the
cooling surface must have been horizontal: the mass must, therefore, have
formed a sheet, and not a dyke; for, in the latter case, the cooling
surfaces would have been vertical.
(3) It is difficult to conceive, on the supposition that the volcanic rock
is a neck with perpendicular sides, that the marine denudation should have
uniformly proceeded only so far as to lay bare the junction between the two
formations. We should have expected that in many places the igneous rock
itself would have been cut down to the general level, whereas the only
signs of such an effect are shown in a few narrow inlets where the rock was
manifestly softer than in the surrounding parts.
The last objection is greatly confirmed by the overhanging cliffs and
numerous blocks of porphyrite which lie scattered on the beach, as if to
attest the former extension of that ancient sheet of which these blocks now
form but a small remnant. Indeed, the existence of such remains appears
sufficient of itself to condemn any hypothesis which presumes the present
face of the cliff to have formed the original boundary of the mass.
It may be fairly objected to our theory, as Prof. Geikie himself has
suggested, that the high angle at which the strata dip is difficult to
account for. But, in fact, this steep inclination constitutes the very
difficulty which any hypothesis on the subject must be framed to explain;
and it is a difficulty which is not more easily solved by Prof. Geikie's
theory than by our own.
II. THE DEVELOPMENT AND GROWTH OF THE LAYERS OF THE BLASTODERM[6].
Footnote 6: From the _Quarterly Journal of Microscopical
Science_, Vol. XIII., 1873.
With Plate 1, figs. 1-5 and 9-12.
The following paper deals with the changes which take place in the cells of
the blastoderm of the hen's egg during the first thirty or forty hours of
incubation. The subject is one which has, as a general rule, not been much
followed up by embryologists, but is nevertheless of the greatest interest,
both in reference to embryology itself, and to the growth and changes of
protoplasm exhibited in simple embryonic cells. I am far from having
exhausted the subject in this paper, and in some cases I shall be able
merely to state facts, without being able to give any explanation of their
meaning.
My method of investigation has been the examination of sections and surface
views. For hardening the blastoderm I have employed, as usual, chromic
acid, and also gold chloride. It is, however, difficult to make sections of
blastoderms hardened by this latter reagent, and the sections when made are
not in all cases satisfactory. For surface views I have chiefly used silver
nitrate, which brings out the outlines of the cells in a manner which
leaves nothing to be desired as to clearness. If the outlines only of the
cells are to be examined, a very short immersion (half a minute) of the
blastoderm in a half per cent. solution of silver nitrate is sufficient,
but if the immersion lasts for a longer period the nuclei will be brought
out also. For studying the latter, however, I have found it better to
employ gold chloride or carmine in conjunction with the silver nitrate.
My observations begin with the blastoderm of a freshly laid egg. The
appearances presented by sections of this have been accurately described by
Peremeschko, "Ueber die Bildung der Keimblätter im Hühnerei,"
_Sitzungsberichte der K. Akademie der Wissenschaften in Wien_, 1868.
Oellacher, "Untersuchung über die Furchung und Blatterbildung im Hühnerei,"
_Studien aus dem Institut für Experim. Pathologie in Wien_, 1870 (pp.
54-74), and Dr Klein, lxiii. Bande der _Sitz. der K. Acadamie der Wiss. in
Wien_, 1871.
The unincubated blastoderm (Pl. 1, fig. 1) consists of two layers. The
upper layer is composed of a single row of columnar cells. Occasionally,
however, the layer may be two cells thick. The cells are filled with highly
refracting spherules of a very small size, and similar in appearance to the
finest white yolk spherules, and each cell also contains a distinct oval
nucleus. This membrane rests with its extreme edge on the white yolk, its
central portion covering in the segmentation cavity. From the very first it
is a distinct coherent membrane, and exhibits with silver nitrate a
beautiful hexagonal mosaic of the outlines (Pl. 1, fig. 6) of the cells.
The diameter of the cells when viewed from above is from 1/2000 - 1/3000 of
an inch. The under layer is very different from this: it is composed of
cells which are slightly, if at all, united, and which vary in size and
appearance, and in which a nucleus can rarely be seen. The cells of which
it is composed fill up irregularly the segmentation cavity, though a
distinct space is even at this time occasionally to be found at the bottom
of it. Later, when the blastoderm has spread and the white yolk floor has
been used as food, a considerable space filled with fluid may generally be
found.
The shape of the floor of the cavity varies considerably, but it is usually
raised in the middle and depressed near the circumference. In this case the
under layer is perhaps only two cells deep at the centre and three or four
cells deep near the circumference.
The cells of which this layer is composed vary a good deal in size; the
larger cells being, however, more numerous in the lower layers. In
addition, there are usually a few very large cells quite at the bottom of
the cavity, occasionally separated from the other cells by fluid. They were
called _formative cells_ (Bildungselemente) by Peremeschko (_loc. cit._);
and, according to Oellacher's observations (_loc. cit._), some of them, at
any rate, fall to the bottom of the segmentation cavity during the later
stages of segmentation. They do not differ from the general lower layer
cells except in size, and even pass into them by insensible gradations. All
the cells of the lower layer are granular, and are filled with highly
refracting spherules precisely similar to the smaller white yolk spherules
which line the bottom of the segmentation cavity.
The size of the ordinary cells of the lower layer varies from 1/2000 -
1/1000 of an inch. The largest of the formative cells come up to 1/300 of
an inch. It will be seen from this description that, morphologically
speaking, we cannot attach much importance to the formative cells. The fact
that they broke off from the blastoderm, towards the end of the
segmentation--even if we accept it as a normal occurrence, rather than the
result of manipulation--is not of much importance, and, except in size, it
is impossible to distinguish these cells from other cells of the lower
layer of the blastoderm.
Physiologically, however, as will be afterwards shewn, they are of
considerable importance.
The changes which the blastoderm undergoes during the first three or four
hours of incubation are not very noticeable. At about the sixth or eighth
hour, or in some cases considerably earlier, changes begin to take place
very rapidly. These changes result in the formation of a hypoblast and
mesoblast, the upper layer of cells remaining comparatively unaltered as
the epiblast.
To form the hypoblast a certain number of the cells of the lower layer
begin to undergo remarkable changes. From being spherical and, as far as
can be seen, non-nucleated, they become (vide fig. 2, _h_) flattened and
nucleated, still remaining granular, but with fewer spherules.
Here, then, is a direct change, of which all the stages can be followed, of
a cell of one kind into a cell of a totally different character. The new
cell is not formed by a destruction of the old one, but directly from it by
a process of metamorphosis. These hypoblast cells are formed first at the
centre and later at the circumference, so that from the first the cells at
the circumference are less flattened and more granular than the cells at
the centre. A number of cells of the original lower layer are enclosed
between this layer and the epiblast; and, in addition to these, the
formative cells (as has been shewn by Peremeschko, Oellacher, and Klein,
whose observations I can confirm) begin to travel towards the
circumference, and to pass in between the epiblast and hypoblast.
Both the formative cells, and the lower layer cells enclosed between the
hypoblast and epiblast, contribute towards the mesoblast, but the mode in
which the mesoblast is formed is very different from that in which the
hypoblast originates.
It is in this difference of formation that the true distinction between the
mesoblast and hypoblast is to be looked for, rather than in the original
difference of the cells from which they are derived.
The cells of the mesoblast are formed by a process which seems to be a kind
of free cell formation. The whole of the interior of each of the formative
cells, and of the other cells which are enclosed between the epiblast and
the hypoblast, become converted into new cells. These are the cells of the
mesoblast. I have not been able perfectly to satisfy myself as to the exact
manner in which this takes place, but I am inclined to think that some or
all of the spherules which are contained in the original cells develop into
nuclei for the new cells, the protoplasm of the new cells being formed from
that of the original cells.
The stages of formation of the mesoblast cells are shewn in the section
(Pl. 1, fig. 2), taken from the periphery of a blastoderm of eight hours.
The first formation of the mesoblast cells takes place in the centre of the
blastoderm, and the mass of cells so formed produces the opaque line known
as the primitive streak. This is shown in Pl. 1, fig. 9.
One statement I have made in the above description in reference to the
origin of the mesoblast cells, viz. that they are only partly derived from
the formative cells at the bottom of the segmentation cavity, is to a
certain extent opposed to the statements of the three investigators above
mentioned. They state that the mesoblast is entirely derived from the
formative cells. It is not a point to which I attach much importance,
considering that I can detect no difference between these cells and any
other cells of the original lower layer except that of size; and even this
difference is probably to be explained by their proximity to the white
yolk, whose spherules they absorb. But my reason for thinking it probable
that these cells alone do not form the mesoblast are: 1st. That the
mesoblast and hypoblast are formed nearly synchronously, and except at the
centre a fairly even sprinkling of lower layer cells is from the first to
be distinguished between the epiblast and hypoblast. 2nd. That if some of
the lower layer cells are not converted into mesoblast, it is difficult to
see what becomes of them, since they appear to be too numerous to be
converted into the hypoblast alone. 3rd. That the chief formation of
mesoblast at first takes place in the centre, while if the formative cells
alone took part in its formation, it would be natural to expect that it
would begin to be formed at the periphery.
Oellacher himself has shewn (_Zeitschrift für wissenschaftliche Zoologie_,
1873, "Beiträge zur Entwick. Gesch. der Knochenfische") that in osseous
fishes the cells which break away from the blastoderm take no share in the
formation of the mesoblast, so that we can derive no argument from the
formation of the mesoblast in these animals, for believing that in the
chick it is derived only from the formative cells.
In the later stages, however, from the twelfth to the twenty-fifth hour,
the growth of the mesoblast depends almost entirely on these cells, and
Peremeschko's discovery of the fact is of great value.
Waldeyer (_Henle und v. Pfeufer's Zeitschrift_, xxxiv. Band, für 1869) has
given a different account of the origin of the layers. There is no doubt,
however, in opposition to his statements and drawings, that from the very
first the hypoblast is distinct from the mesoblast, which is, indeed, most
conspicuously shewn in good sections; and his drawings of the derivation of
the mesoblast from the epiblast are not very correct.
The changes which have been described are also clearly shewn by means of
silver nitrate. Whereas, at first this reagent brought out no outline
markings of cells in the lower layer, by the eighth to the twelfth hour the
markings (Pl. 1, fig. 3) are very plain, and shew that the hypoblast is a
distinct coherent membrane.
In section, the cells of the hypoblast appear generally very thin and
spindle shaped, but the outlines brought out by the silver nitrate shew
that they are much expanded horizontally, but very irregular as to size,
varying even within a small area from 1/4000 - 1/400 of an inch in the
longest diameter.
At about the twelfth hour they are uniformly smaller a short way from each
extremity of its longer axis than over the rest of the blastoderm.
It is, perhaps, fair to conclude from this that growth is most rapid at
these parts.
At this time the hypoblast, both in sections and from a surface view after
treatment with silver nitrate, appears to end abruptly against the white
yolk. The surface view also shews that its cells are still filled with
highly refractive globules, making it difficult to see the nucleus. In some
cases I thought that I could (fig. 3, _a_) make out that it was hour-glass
shaped, and some cells certainly contain two nuclei. Some of the cells
(fig. 3, _b_) shew re-entrant curves, which prove that they have undergone
division.
The cells of the epiblast, up to the thirteenth hour, have chiefly
undergone change in becoming smaller.
In surface views they are about 1/4000 of an inch in diameter over the
centre of the pellucid area, and increase to 1/2000 of an inch over the
opaque area.
In the centre of the pellucid area the form of the epiblast cells is more
elongated vertically and over the opaque area more flattened than was the
case with the original upper layer cells. In the centre the epiblast is two
or three cells deep.
Before going on to the further changes of the blastodermic cells it will be
well to say a few words in reference to the origin of the mesoblast.
From the description given above it will be clear that in the chick the
mesoblast has an independent origin; it can be said neither to originate
from the epiblast nor from the hypoblast. It is formed coincidently with
the latter out of apparently similar segmentation cells. The hypoblast, as
has been long known, shews in the chick no trace of its primitive method of
formation by involution, neither does the mesoblast shew any signs of its
primitive mode of formation. In so excessively highly differentiated a type
as birds we could hardly expect to find, and certainly do not find, any
traces of the primitive origin of the mesoblast, either from the epiblast
or hypoblast, or from both. In the chick the mesoblast cells are formed
directly from the ultimate products of segmentation. From having a
secondary origin in most invertebrates the mesoblast comes to have, in the
chick, a primary origin from the segmentation spheres, precisely as we find
to be the case with the nervous layer in osseous fishes. It is true we
cannot tell which segmentation-cells will form the mesoblast, and which the
hypoblast; but the mesoblast and hypoblast are formed at the same time, and
both of them directly from segmentation spheres.
The process of formation of the mesoblast in Loligo, as observed by Mr Ray
Lankester (_Annals and Magazine of Natural History_, February, 1873), is
still more modified. Here the mesoblast arises independently of the
blastoderm, and by a process of free cell-formation in the yolk round the
edge of the blastoderm. If Oellacher's observations in reference to the
origin of formative cells are correct, then the modes of origin of the
mesoblast in Loligo and the chick would have nothing in common; but if the
formative cells are in reality derived from the white yolk, and also are
alone concerned in the formation of the mesoblast, then the modes of
formation of the mesoblast in the chick would be substantially the same as
that observed by Mr Ray Lankester in Loligo.
No very important changes take place in the actual forms of the cells
during the next few hours. A kind of fusion takes place between the
epiblast and the mesoblast along the line of the primitive streak forming
the axis-string of His; but the line of junction between the layers is
almost always more or less visible in sections. In any case it does not
appear that there is any derivation of mesoblast cells from the epiblast;
and since the fusion only takes place in the region of the primitive
groove, and not in front, where the medullary groove arises (see succeeding
paper), it cannot be considered of any importance in reference to the
possible origin of the Wolffian duct, &c., from the epiblast (as mooted by
Waldeyer, _Eierstock und Ei_, Leipzig, 1870). The primitive groove, as can
be seen in sections, begins to appear very early, generally before the
twelfth hour. The epiblast spreads rapidly over the white yolk, and the
area pellucida also increases in size.
From the mesoblast forming at first only a small mass of cells, which lies
below the primitive streak, it soon comes to be the most important layer of
the blastoderm. Its growth is effected by means of the formative cells.
These cells are generally not very numerous in an unincubated blastoderm,
but rapidly increase in numbers, probably by division; at the same time
they travel round the edge of, and in some cases through, the hypoblast,
and then become converted in the manner described into mesoblast cells.
They act as carriers of food from the white yolk to the mesoblast till,
after the formation of the vascular area, they are no longer necessary. The
numerous cases in which two nucleoli and even two nuclei can be seen in one
cell prove that the mesoblast cells also increase by division.
The growth of the hypoblast takes place in a very different way. It occurs
by a direct conversion, cell for cell, of the white yolk spheres into
hypoblast cells. This interpretation of the appearances, which I will
describe presently, was first suggested to me by Dr Foster, from an
examination of some of my specimens of about thirty-six hours, prepared
with silver nitrate. Where there is no folding at the junction between the
pellucid and opaque areas, there seems to be a perfect continuity in the
silver markings and a gradual transition in the cells, from what would be
undoubtedly called white yolk spheres, to as undoubted hypoblast cells
(vide Pl. 1, fig. 5). In passing from the opaque to the pellucid areas the
number of white yolk spherules in each cell becomes less, but it is not
till some way into the pellucid area that they quite cease to be present. I
at first thought that this was merely due to the hypoblast cells feeding on
the white yolk sphericles, but the perfect continuity of the cells, and the
perfect gradation in passing from the white yolk cells to the hypoblast,
proves that the other interpretation is the correct one, viz. that the
white yolk spheres become directly converted into the hypoblast cells. This
is well shewn in sections (vide Pl. 1, fig. 4) taken from embryos of all
ages from the fifteenth to the thirty-sixth hour and onwards. But it is,
perhaps, most easily seen in embryos of about twenty hours. In such an
embryo there is a most perfect gradation: the cells of the hypoblast
become, as they approach the edge of the pellucid area, broader, and are
more and more filled with white yolk sphericles, till at the line of
junction it is quite impossible to say whether a particular cell is a
white-yolk cell (sphere) or a hypoblast cell. The white-yolk cells near the
line of junction can frequently be seen to possess nuclei. At first the
hypoblast appears to end abruptly against the white yolk; this state of
things, however, soon ends, and there supervenes a complete and unbroken
continuity between the hypoblast and the white yolk.
Of the mode of increase of the epiblast I have but little to say. The cells
undoubtedly increase entirely by division, and the new material is most
probably derived directly from the white yolk.
Up to the sixth hour the cells of the upper layer retain their early
regular hexagonal pattern, but by the twelfth hour they have generally
entirely lost this, and are irregularly shaped and very angular. The cells
over the centre of the pellucid area remain the smallest up to the
twenty-fifth hour or later, while those over the rest of the pellucid area
are uniformly larger.
In the hypoblast the cells under the primitive groove, and on each side as
far as the fold which marks off the exterior limit of the protovertebræ are
at the eighteenth hour considerably smaller than any other cells of this
layer.
In all the embryos between the eighteenth and twenty-third hour which I
have examined for the purpose, I have found that at about two-thirds of the
distance from the anterior end of the pellucid area, and just external to
the side fold, there is a small space on each side in which the cells are
considerably larger than anywhere else in the hypoblast. These larger
cells, moreover, contain a greater number of highly refractive spherules
than any other cells. It is not easy to understand why growth should have
been less rapid here than elsewhere, as the position does not seem to
correspond to any feature in the embryo. In some specimens the hypoblast
cells at the extreme edge of the pellucid area are smaller than the cells
immediately internal to them. At about the twenty-third hour these cells
begin rapidly to lose the refractive spherules they contained in the
earlier stages of incubation, and come to consist of a nucleus surrounded
simply by granular protoplasm.
At about this period of incubation the formative cells are especially
numerous at the periphery of the blastoderm, and, no doubt, become
converted into the mass of mesoblast which is found at about the
twenty-fifth hour in the region of the vascular area. Some of them are
lobate, and appear as if they were undergoing division. At this time also
the greatest number of formative cells are to be found at the bottom of the
now large segmentation cavity.
In embryos of from thirty to forty hours the cells of the hypoblast have,
over the central portion of the pellucid area, entirely lost their highly
refractive spherules, and in the fresh state are composed of the most
transparent protoplasm. When treated with reagents they are found to
contain an oval nucleus with one or sometimes two nucleoli, imbedded in a
considerable mass of protoplasm. The protoplasm appears slightly granular
and generally contains one or two small vacuoles. I have already spoken of
the gradation of the hypoblast at the edge of the blastoderm into white
yolk. I have, therefore, only to mention the variations in the size of its
cells in different parts of the pellucid area. The points where the cells
are smallest seem generally to coincide with the points of maximum growth.
Over the embryo the cells are more regular than elsewhere. They are
elongated and arranged transversely to the long axis of the embryo. They
are somewhat hexagonal in shape, and not unlike the longer pieces in the
dental plate of a Myliobatis (Pl. 1, fig. 10). This regularity, however, is
much more marked in some specimens than in others. These cells are about
1/4000th of an inch in breadth, and 1/1000th in length. On each side of the
embryo immediately external to the protovertebræ the cells are frequently
about the same size as those over the embryo itself. In the neck, however,
and near the end of the sinus rhomboidalis, they are considerably smaller,
about 1/4000th inch each way. The reason of this small size is not very
clear, but probably shews that the greatest growth is taking place at these
two points. The cells, again, are very small at the head fold, but are very
much larger in front of this--larger, in fact, than any other cells of the
hypoblast. Outside the embryo they gradually increase in size towards the
edge of the pellucid area. Here they are about 1/1000th of an inch in
diameter, irregular in shape and rather angular.
The outlines of the cells of the epiblast at this time are easily
distinguished from the cells of the hypoblast by being more elongated and
angular; they are further distinguished by the presence of numerous small
oval cells, frequently at the meeting point of several cells, at other
times at points along the lines of junction of two cells (Pl. 1, fig. 12).
These small cells look very like the smaller stomata of endothelial
membranes, but are shewn to be cells by possessing a nucleus. There is
considerable variation in size in the cells in different parts of the
epiblast. Between the front lobes of the brain the cells are very small,
1/4000th inch, rising to 1/2000th on each side. They are about the latter
size over the greater part of the embryo. But over the sinus rhomboidalis
they fall again to from 1/3000th to 1/4000th inch. This is probably to be
explained by the growth of the medullary fold at this point, which pushes
back the primitive groove. At the sides of the head the cells are larger
than anywhere else in the epiblast, being here about 1/1000th inch in
diameter. I at present see no explanation of this fact. At the periphery of
the pellucid area and over the vascular area the cells are 1/1500th to
1/2000th inch in diameter, but at the periphery of the opaque area they are
smaller again, being about the 1/3000th of an inch. This smaller size at
the periphery of the area opaca is remarkable, since in the earlier stages
the most peripheral epiblast cells were the largest. It, perhaps, implies
that more rapid growth is at this time taking place in that part of the
epiblast which is spreading over the yolk sac.
EXPLANATION OF PLATE 1, Figs. 1-5 and 9-12.
Fig. 1. Section through an unincubated blastoderm, shewing the upper layer,
composed of a single row of columnar cells, and the lower layer, composed
of several rows of rounded cells in which no nucleus is visible. Some of
the "formative cells," at the bottom of the segmentation cavity, are seen
at (_b_).
Fig. 2. Section through the periphery of an eight hours' blastoderm,
shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast
commencing to be formed (_c_), partly by lower-layer cells enclosed between
the epiblast and hypoblast, and partly by formative cells. Formative cells
at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of
the side folds parallel to the primitive groove.
Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated
with silver nitrate, shewing the great variation in the size of the cells
at this period. An hour-glass shaped nucleus is seen at _a_.
Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for
cell the junction between the hypoblast (_h_) and white-yolk spheres (_w_).
Fig. 5. Junction between the white-yolk spheres and the hypoblast cells at
the passage from the area pellucida to the area opaca. The specimen was
treated with silver nitrate to bring out the shape of the cells. The line
of junction between the opaque and pellucid areas passes diagonally.
Fig. 9. Section through the primitive streak of an eight hours' blastoderm.
The specimen shews the mesoblast very much thickened in the immediate
neighbourhood of the primitive streak, but hardly formed at all on each
side of the streak. It also shews the primitive groove just beginning to be
formed (_pr_), and the fusion between the epiblast and the mesoblast under
the primitive groove. The hypoblast is completely formed in the central
part of the blastoderm. At _f_ is seen one of the side folds parallel to
the primitive groove. Its depth has been increased by the action of the
chromic acid.
Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo,
treated with silver nitrate, shewing the regularity and elongated shape of
the cells over the embryo and the smaller cells on each side.
Fig. 11. Epiblast cells from an unincubated blastoderm, treated with silver
nitrate, shewing the regular hexagonal shape of the cells and the small
spherules they contain.
Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated
with silver nitrate, shewing the small rounded cells frequently found at
the meeting-points of several larger cells which are characteristic of the
upper layer.
III. ON THE DISAPPEARANCE OF THE PRIMITIVE GROOVE IN THE EMBRYO CHICK[7].
Footnote 7: From the _Quarterly Journal of Microscopical
Science_, Vol. XIII, 1873.
With Plate 1, Figs. 6-8 and 13-19.
The investigations of Dursy (_Der Primitivstreif des Hühnchens_, von Dr E.
Dursy. Lahr, 1866) on the primitive groove, shewing that it is a temporary
structure, and not connected with the development of the neural canal, have
in this country either been ignored or rejected. They are, nevertheless,
perfectly accurate; and had Dursy made use of sections to support his
statements I do not think they would so long have been denied. In Germany,
it is true, Waldeyer has accepted them with a few modifications, but I have
never seen them even alluded to in any English work. The observations which
I have made corroborating Dr Dursy may, perhaps, under these circumstances
be worth recording.
After about twelve hours of incubation the pellucid area of a hen's egg has
become somewhat oval, with its longer axis at right angles to the long axis
of the egg. Rather towards the hinder (narrower) end of this an opaque
streak has appeared, with a somewhat lighter line in the centre. A section
made at the time shews that the opaque streak is due partly to a thickening
of the epiblast, but more especially to a large collection of the rounded
mesoblast cells, which along this opaque line form a thick mass between the
epiblast and the hypoblast. The mesoblast cells are in contact with both
hypoblast and epiblast, and appear to be fused with the latter. The line of
junction between them can, however, almost always be made out.
Soon after the formation of this primitive streak a groove is formed along
its central line by a pushing inwards of the epiblast. The epiblast is not
thinner where it lines the groove, but the mass of mesoblast below the
groove is considerably thinner than at its two sides. This it is which
produces the peculiar appearance of the primitive groove when the
blastoderm is viewed by transmitted light as a transparent line in the
middle of an opaque one.
This groove, as I said above, is placed at right angles to the long axis of
the egg, and nearer the hind end, that is, the narrower end of the pellucid
area. It was called "the primitive groove" by the early embryologists, and
they supposed that the neural canal arose from the closure of its edges
above. It is always easy to distinguish this groove, in transverse
sections, by several well-marked characters. In the first place, the
epiblast and mesoblast always appear more or less fused together underneath
it; in the second place, the epiblast does not become thinner where it
lines the groove; and in the third place, the mesoblast beneath it never
shews any signs of being differentiated into any organ.
As Dursy has pointed out, there is frequently to be seen in fresh
specimens, examined as transparent objects, a narrow opaque line running
down the centre of this groove. I do not know what this line is caused by,
as there does not appear to be any structural feature visible in sections
to which it can correspond.
From the twelfth to the sixteenth hour the primitive groove grows rapidly,
and by the sixteenth hour is both absolutely and considerably longer than
it was at the twelfth hour, and also proportionately longer as compared
with the length of the pellucid area.
There is a greater interval between its end and that of the pellucid area
in front than behind.
At about the sixteenth hour, or a little later, a thickening of the
mesoblast takes place in front of the primitive groove, forming an opaque
streak, which in fresh specimens looks like a continuation from the
anterior extremity of the primitive groove (vide Pl. 1, fig. 8). From
hardened specimens, however, it is easy to see that the connection of this
streak with the primitive groove is only an apparent one. Again, it is
generally possible to see that in the central line of this streak there is
a narrow groove. I do not feel certain that there is no period when this
groove may not be present, but its very early appearance has not been
recognized either by Dursy or by Waldeyer. Moreover, both these authors, as
also His, seem to have mistaken the opaque streak spoken of above for the
notochord. This, however, is not the case, and the notochord does not make
its appearance till somewhat later. The mistake is of very minor
importance, and probably arose in Dursy's case from his not sufficiently
making use of sections. At about the time the streak in front of the
primitive groove makes its appearance a semicircular fold begins to be
formed near the anterior extremity of the pellucid area, against which the
opaque streak, or as it had, perhaps, better be called, "the medullary
streak," ends abruptly.
This fold is the head fold, and the groove along the medullary streak is
the medullary groove, which subsequently forms the cavity of the medullary
or neural canal.
Everything which I have described above can without difficulty be made out
from the examination of fresh and hardened specimens under the simple
microscope; but sections bring out still more clearly these points, and
also shew other features which could not have been brought to light without
their aid. In Pl. 1, figs. 6 and 7, two sections of an embryo of about
eighteen hours are shewn. The first of these passes through the medullary
groove, and the second of them through the extreme anterior end of the
primitive groove. The points of difference in the two sections are very
obvious.
From fig. 6 it is clear that a groove has already been formed in the
medullary streak, a fact which was not obvious in the fresh specimen. In
the second place the mesoblast is thickened both under the groove and also
more especially in the medullary folds at the sides of the groove; but
shews hardly a sign of the differentiation of the notochord. So that it is
clear that the medullary streak is not the notochord, as was thought to be
the case by the authors above mentioned. In the third place there is no
adhesion between the epiblast and the mesoblast. In all the sections I have
cut through the medullary groove I have found this feature to be constant;
while (for instance, as in Pl. 1, figs. 7, 9, 17) all sections through the
primitive groove shew most clearly an adhesion between the epiblast and
mesoblast. This fact is both strongly confirmatory of the separate origins
of the medullary and primitive grooves, and is also important in itself, as
leaving no loophole for supposing that in the region of embryo there is any
separation of the cells from the epiblast to form the mesoblast.
By this time the primitive groove has attained its maximum growth, and from
this time begins both absolutely to become smaller, and also gradually to
be pushed more and more backwards by the growth of the medullary groove.
The specimen figured in Pl. 1, fig. 18, magnified about ten diameters,
shews the appearance presented by an embryo of twenty-three hours. The
medullary groove (_mc_) has become much wider and deeper than it was in the
earlier stage; the medullary folds (_A_) are also broader and more
conspicuous. The medullary groove widens very much posteriorly, and also
the medullary folds separate far apart to enclose the anterior end of the
primitive groove (_pr_).
All this can easily be seen with a simple microscope, but the sections
taken from the specimen figured also fully bear out the interpretations
given above, and at the same time shew that the notochord has at this age
begun to appear. The sections marked 13-17 pass respectively through the
lines with corresponding numbers in fig. 18. Section 1 (fig. 13) passes
through the middle of the medullary canal.
In it the following points are to be noted. (1) That the epiblast becomes
very much thinner where it lines the medullary canal (_mc_), a feature
never found in the epiblast lining the primitive groove. (2) That the
mesoblast is very much thickened to form the medullary folds at _A_, _A_,
while there is no adherence between it and the epiblast, below the
primitive groove. (3) The notochord (_ch_) has begun to be formed, though
its separation from the rest of the mesoblast is not as yet very
distinct[8].
Footnote 8: In the figure the notochord has been made too
distinct.
In fig. 14 the medullary groove has become wider and the medullary folds
broader, the notochord has also become more expanded: the other features
are the same as in section 1. In the third section (fig. 15) the notochord
is still more expanded; the bottom of the now much expanded medullary
groove has become raised to form the ridge which separates the medullary
from the primitive groove. The medullary folds are also flatter and broader
than in the previous section. Section 4 (fig. 16) passes through the
anterior end of the primitive groove. Here the notochord is no longer
visible, and the adherence between the mesoblast and epiblast below the
primitive groove comes out in marked contrast with the entire separation of
the two layers in the previous sections.
The medullary folds (_A_) are still visible outside the raised edges of the
primitive groove, and are as distinctly as possible separate and
independent formations, having no connection with the folds of the
primitive groove. In the last section (fig. 17), which is taken some way
behind section 4, no trace of the medullary folds is any longer to be seen,
and the primitive groove has become deeper. This series of sections, taken
in conjunction with the specimen figured in fig. 18, must remove all
possible doubt as to the total and entire independence of the primitive and
medullary grooves. They arise in different parts of the blastoderm; the one
reaches its maximum growth before the other has commenced to be formed; and
finally, they are distinguished by almost every possible feature by which
two such grooves could be distinguished.
Soon after the formation of the notochord, the protovertebræ begin to be
formed along the sides of the medullary groove (Pl. 1, fig. 19, _pv_). Each
new protovertebra (of those which are formed from before backwards) arises
just in front of the anterior end of the primitive groove. As growth
continues, the primitive groove becomes pushed further and further back,
and becomes less and less conspicuous, till at about thirty-six hours only
a very small and curved remnant is to be seen behind the sinus
rhomboidalis; but even up to the forty-ninth Dursy has been able to
distinguish it at the hinder end of the embryo.
The primitive groove in the chick is, then, a structure which appears very
early, and soon disappears without entering directly into the formation of
any part of the future animal, and without, so far as I can see, any
function whatever. It is clear, therefore, that the primitive groove must
be the rudiment of some ancestral feature; but whether it is a rudiment of
some structure which is to be found in reptiles, or whether of some earlier
form, I am unable to decide. It is just possible that it is the last trace
of that involution of the epiblast by which the hypoblast is formed in most
of the lower animals. The fact that it is formed in the hinder part of the
pellucid area perhaps tells slightly in favour of this hypothesis, since
the point of involution of the epiblast not unfrequently corresponds with
the position of the anus.
EXPLANATION OF PLATE 1, Figs. 6-8 and 13-19.
Figs. 6 and 7 are sections through an embryo rather earlier than the one
drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove
(_md_), which appears in fresh specimens, as in fig. 8, merely as an opaque
streak coming from the end of the primitive groove. The notochord is hardly
differentiated, but the _complete_ separation of mesoblast and hypoblast
under the primitive groove is clearly shewn. Fig. 7 passes through the
anterior end of the primitive groove (_pr_), and shews the fusion between
the mesoblast and epiblast, which is always to be found under the primitive
groove.
Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a
transparent object. Primitive groove (_pr_). Medullary groove (_md_), which
passes off from the anterior end of the primitive groove, and is produced
by the thickening of the mesoblast. Head fold (_pf_).
Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through
the lines 1, 2, 3, 4, 5 respectively.
The first section (fig. 13) passes through the true medullary groove
(_mc_); the two medullary folds (_A_, _A_) are seen on each side with the
thickened mesoblast, and the mesoblast cells are beginning to form the
notochord (_nc_) under the medullary groove. There is no adherence between
the mesoblast cells and the epiblast under the medullary groove.
The second (fig. 14) section passes through the medullary groove where it
has become wider. Medullary folds, _A_, _A_; notochord, _ch_.
In the third section (fig. 15) the notochord (_ch_) is broader, and the
epiblast is raised in the centre, while the medullary folds are seen far
apart at _A_.
In section fig. 16 the medullary folds (_A_) are still to be seen enclosing
the anterior end of the primitive groove (_pr_). Where the primitive groove
appears there is a fusion of the epiblast and mesoblast, and no appearance
of the notochord.
In the last section, fig. 17, no trace is to be seen of the medullary
folds.
Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is
twenty-three hours old; fig. 19 twenty-five hours. They both shew how the
medullary canal arises entirely independently of the primitive groove and
in front of it, and also how the primitive groove gets pushed backwards by
the growth of the medullary groove. _pv_, Protovertebræ; other references
as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were
cut.
IV. THE DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK[9].
Footnote 9: From the _Quarterly Journal of Microscopical
Science_, Vol. XIII, 1873.
With Plate 2.
The development of the first blood-vessels of the yolk-sac of the chick has
been investigated by a large number of observers, but with very discordant
results. A good historical _résumé_ of the subject will be found in a paper
of Dr Klein (liii. Band der _K. Akad. der Wissensch. Wien_), its last
investigator.
The subject is an important one in reference to the homologies of the
blood-vascular system of the vertebrata. As I shall shew in the sequel (and
on this point my observations agree with those of Dr Klein), the
blood-vessels of the chick do not arise as spaces or channels between the
cells of the mesoblast; on the contrary, they arise as a network formed by
the united processes of mesoblast-cells, and it is through these processes,
and not in the spaces between them, that the blood flows. It is, perhaps,
doubtful whether a system of vessels arising in this way can be considered
homologous with any vascular system which takes its origin from channels
hollowed out in between the cells of the mesoblast.
My own researches chiefly refer to the development of the blood-vessels in
the pellucid area. I have worked but very slightly at their development in
the vascular area; but, as far as my observations go, they tend to prove
that the mode of their origin is the same, both for the pellucid and the
vascular area.
The method which I have principally pursued has been to examine the
blastoderm from the under surface. It is very difficult to obtain exact
notions of the mode of development of the blood-vessels by means of
sections, though these come in as a valuable confirmation of the other
method.
For the purpose of examination I have employed (1) fresh specimens; (2)
specimens treated with spirit, and then mounted in glycerine; (3) specimens
treated with chloride of gold for about half a minute, and then mounted in
glycerine; and (4) specimens treated with osmic acid.
All these methods bring out the same appearances with varying clearness;
but the successful preparations made by means of the gold chloride are the
best, and bring out the appearances with the greatest distinctness.
The first traces of the blood-vessels which I have been able to distinguish
in the pellucid area are to be seen at about the thirtieth hour or slightly
earlier, at about the time when there are four to five protovertebræ on
each side.
Fig. 1 shews the appearance at this time. Immediately above the hypoblast
there are certain cells whose protoplasm sends out numerous processes.
These processes vary considerably in thickness and size, and quickly come
in contact with similar processes from other cells, and unite with them.
I have convinced myself, by the use of the hot stage, that these processes
continually undergo alteration, sometimes uniting with other processes,
sometimes becoming either more elongated and narrower or broader and
shorter. In this way a network of somewhat granular protoplasm is formed
with nuclei at the points from which the processes start.
From the first a difference may be observed in the character of this
network in different parts of the pellucid area. In the anterior part the
processes are less numerous and thicker, the nuclei fewer, and the meshes
larger; while in the posterior part the processes are generally very
numerous, and at first thin, the meshes small, and the nuclei more
frequent. As soon as this network commences to be formed the nuclei begin
to divide. I have watched this take place with the hot stage. It begins by
the elongation of the nucleus and division of the nucleolus, the parts of
which soon come to occupy the two ends of the nucleus. The nucleus becomes
still longer and then narrows in the centre and divides. By this means the
nuclei become much more numerous, and are found in almost all the larger
processes. Whether they are carried out into the processes by the movement
of the surrounding protoplasm, or whether they move through the protoplasm,
I have been unable to determine; the former view, however, seems to be the
most probable.
It is possible that some nuclei arise spontaneously in the protoplasm, but
I am much more inclined to think that they are all formed by the division
of pre-existing nuclei--a view favoured by the number of nuclei which are
seen to possess two nucleoli. Coincidently with the formation of the new
nuclei the protoplasm of the processes, as well as that surrounding the
nuclei at the starting-points of the processes, begins to increase in
quantity.
At these points the nuclei also increase more rapidly than elsewhere, but
at first the resulting nuclei seem to be all of the same kind.
In the anterior part of the pellucid area (fig. 4) the increase in the
number of nuclei and in the amount of protoplasm at the starting-points of
the protoplasm is not very great, but in the posterior part the increase in
the amount of the protoplasm at these points is very marked, and
coincidently the increase in number of the nuclei is also great. This is
shewn in figs. 2 and 3. These are both taken from the tail end of an embryo
of about thirty-three hours, with seven or eight protovertebræ. Fig. 3
shews the processes beginning to increase in thickness, and also the
protoplasm at the starting-points increasing in quantity; at the same time
the nuclei at these points are beginning to become more numerous. Fig. 3 is
taken from a slightly higher level, _i.e._ slightly nearer the epiblast. In
it the protoplasm is seen to have increased still more in quantity, and to
be filled with nuclei. These nuclei have begun to be slightly coloured, and
one of them is seen to possess two nucleoli.
Very soon after this a change in the nuclei begins to be observed, more
especially in the hinder part of the embryo. While before this time they
were generally elongated, some of them now become more nearly circular. In
addition to this, they begin to have a yellowish tinge, and the nuclei,
when treated with gold (for in the fresh condition it is not easy to see
them distinctly), have a more jagged and irregular appearance than the
nucleoli of the other nuclei.
This change takes place especially at the starting-points of the processes,
so that the appearance presented (fig. 5) is that of spherical masses of
yellowish nuclei connected with other similar spherical masses by
protoplasmic processes, in which nuclei of the original type are seen
imbedded. These masses are surrounded by a thin layer of protoplasm, at the
edge of which a normal nucleus may here and there be detected, as at
fig. 5, _a_ and _a´_, the latter possessing two nucleoli. Some of these
processes are still very delicate, and it is exceedingly probable that they
undergo further changes of position before the final capillary system is
formed.
These differentiated nuclei are the first stage in the formation of the
blood-corpuscles. From their mode of formation it is clear that the
blood-corpuscles of the Sauropsida are to be looked upon as nuclei
containing nucleoli, rather than as cells containing nuclei; indeed, they
seem to be merely ordinary nuclei with red colouring matter.
This would make them truly instead of only functionally homologous with the
red corpuscles of the Mammalia, and would well agree with the fact that the
red corpuscles of Mammalia, in their embryonic condition, possess what have
previously been called nuclei, but which might perhaps more properly be
called nucleoli.
In the anterior part of the blastoderm the processes, as I have stated, are
longer and thinner, and the spaces enclosed between them are larger. This
is clearly brought out in Pl. 2, fig. 4. But, besides these large spaces,
there are other smaller spaces, such as that at _v_. It is, on account of
the transparency of the protoplasm, very difficult to decide whether these
are vacuoles or simply spaces enclosed by the processes, but I am inclined
to think that they are merely spaces. The difficulty of exactly determining
this point is increased by the presence of numerous white-yolk spherules in
the hypoblast above, which considerably obscure the view. At about the same
time that the blood-corpuscles appear in the posterior end of the pellucid
area, or frequently a little later, they begin to be formed in the anterior
part also. The masses of them are, however, far smaller and far fewer than
in the posterior part of the embryo. It is at the tail end of the pellucid
area that the chief formation of blood-corpuscles takes place.
The part of the pellucid area intermediate in position between the anterior
and posterior ends of the embryo is likewise intermediate as regards the
number of corpuscles formed and the size of the spaces between the
processes; the spaces being here larger than at the posterior extremity,
but smaller than the spaces in front. Close to the sides of the embryo the
spaces are, however, smaller than in any other part of the pellucid area.
It is, however, in this part that the first formation of blood-corpuscles
takes place, and that the first complete capillaries are formed.
We have then somewhat round protoplasmic masses filled with
blood-corpuscles and connected by means of processes, a few of which may
begin to contain blood-corpuscles, but the majority of which only contain
ordinary nuclei. The next changes to be noticed take place in the nuclei
which were not converted into blood-corpuscles, but which were to be seen
in the protoplasm surrounding the corpuscles. They become more numerous and
smaller, and, uniting with the protoplasm in which they were imbedded,
become converted into flat cells (spindle-shaped in section), and in a
short time form an entire investment for the masses of blood-corpuscles.
The same change also occurs in the protoplasmic processes which connect the
masses of corpuscles. In the case of those processes which contain no
corpuscles the greater part of their protoplasm seems to be converted into
the protoplasm of the spindle-shaped cells. The nuclei arrange themselves
so as completely to surround the exterior of the protoplasmic processes. In
this way each process becomes converted into a hollow tube, completely
closed in by cells formed from the investment of the original nuclei by the
protoplasm which previously formed the solid processes. The remainder of
the protoplasm probably becomes fluid, and afterwards forms the plasma in
which the corpuscles float. While these changes are taking place the
formation of the blood-corpuscles does not stand still, and by the time a
system of vessels, enclosed by cellular walls, is formed out of the
protoplasmic network, a large number of the connecting processes in this
network have become filled with blood-corpuscles. The appearances presented
by the network at a slightly later stage than this is shewn in Pl. 2,
fig. 6, but in this figure all the processes are seen to be filled with
blood-corpuscles.
This investment of the masses of corpuscles by a cellular wall occurs much
earlier in some specimens than in others, both in relation to the time of
incubation and to the completion of the network. It is generally completed
in some parts by the time there are eight or nine protovertebræ, and is
almost always formed over a great part of the pellucid area by the
thirty-sixth hour. The formation of the corpuscles, as was pointed out
above, occurs earliest in the central part of the hour-glass shaped
pellucid area, and latest in its anterior part. In the hinder part of the
pellucid area the processes, as well as their enlarged starting-points,
become entirely filled with corpuscles; this, however, is by no means the
case in its anterior part. Here, although the corpuscles are undoubtedly
developed in parts as shewn in fig. 7, yet a large number of the processes
are entirely without them. Their development, moreover, is in many cases
very much later. When the development has reached the stage described, very
little is required to complete the capillary system. There are always, of
course, a certain number of the processes which end blindly, and others are
late in their development, and are not by this time opened; but, as a
general rule, when the cellular investment is formed for the masses of
corpuscles, there is completed an open network of tubes with cellular
walls, which are more or less filled with corpuscles. These become quickly
driven into the opaque area in which at that time more corpuscles may
almost always be seen than in the pellucid area.
By the formation of a network of this kind it is clear that there must
result spaces enclosed between the walls of the capillaries; these spaces
have under the microscope somewhat the appearance of being vesicles
enclosed by walls formed of spindle-shaped cells. In reality they are only
spaces enclosed at the sides, and, as a general rule, not above and below.
They have been mistaken by some observers for vesicles in which the
corpuscles were supposed to be developed, and to escape by the rupture of
the walls into the capillary spaces between. This mistake has been clearly
pointed out by Klein (_loc. cit._).
At the time when these spaces are formed, and especially in the hinder
two-thirds of the pellucid area, and in the layer of blood-vessels
immediately above the hypoblast, a formation takes place which forms in
appearance a secondary investment of the capillaries. Dr Klein was the
first to give a correct account of this formation. It results from the
cells of the mesoblast in the meshes of the capillary system. Certain of
these cells become flattened, and send out fine protoplasmic processes.
They arrange themselves so as completely to enclose the spaces between the
capillaries, forming in this way vesicles.
Where seen on section (vide fig. 6) at the edge of the vesicles these cells
lining the vesicles appear spindle-shaped, and look like a secondary
investment of the capillaries. This investment is most noticeable in the
hinder two-thirds of the pellucid area; but, though less conspicuous, there
is a similar formation in its anterior third, where there would seem to be
only veins present. Dr Klein (_loc. cit._, fig. 12) has also drawn this
investment in the anterior third of the pellucid area. He has stated that
the vessels in the mesoblast between the splanchnopleure and the
somatopleure, and which are enclosed by prolongations from the former, do
not possess this secondary investment; he has also stated that the same is
true for the sinus terminalis; but I am rather doubtful whether the
generalisation will hold, that veins and arteries can from the first be
distinguished by the latter possessing this investment. I am also rather
doubtful whether the spaces enclosed by the protoplasmic threads between
the splanchnopleure and somatopleure are the centres of vessels at all,
since I have never seen any blood-corpuscles in them.
It is not easy to learn from sections much about the first stages in the
formation of the capillaries, and it is impossible to distinguish between a
completely-formed vessel and a mere spherical space. The fine protoplasmic
processes which connect the masses of corpuscles can rarely be seen in
sections, except when they pass vertically, as they do occasionally (vide
Pl. 2, fig. 9) in the opaque area, joining the somatopleure and the
splanchnopleure. Dr Klein considers these latter processes to be the walls
of the vessels, but they appear rather to be the processes which will
eventually become new capillaries.
From sections, however, it is easy to see that the appearances of the
capillaries in the vascular area are similar to the appearances in the
pellucid area, from which it is fair to conclude that their mode of
formation is the same in both. It is also easy to see that the first
formation of vessels occurs in the splanchnopleure, and that even up to the
forty-fifth hour but few or no vessels are found in the somatopleure. The
mesoblast of the somatopleure is continued into the opaque area as a single
layer of spindle-shaped cells.
Sections clearly shew in the case of most of the vessels that the secondary
investment of Klein is present, even in the case of those vessels which lie
immediately under the somatopleure.
In reference to the origin of particular vessels I have not much to say. Dr
Klein's account of the origin of the sinus terminalis is quite correct. It
arises by a number of the masses of blood-corpuscles, similar to those
described above, becoming connected together by protoplasmic processes. The
whole is subsequently converted into a continuous vessel in the usual way.
From the first the sinus terminalis possesses cellular walls, as is clear
from its mode of origin. I am inclined to think that Klein is right in
saying that the aortæ arise in a similar manner, but I have not worked out
their mode of origin very fully.
It will be seen from the account given above that, in reference to the
first stages in the development of the blood-vessels, my observations
differ very considerably from those of Dr Klein; as to the later stages,
however, we are in tolerable agreement. We are in agreement, moreover, as
to the fundamental fact that the blood-vessels are formed by a number of
cells becoming connected, and by a series of changes converted into a
network of vessels, and that they are not in the first instance merely
channels between the cells of the mesoblast.
By the forty-fifth hour colourless corpuscles are to be found in the blood
whose exact origin I could not determine; probably they come from the walls
of the capillaries.
In the vessels themselves the coloured corpuscles undergo increase by
division, as has already been shewn by Remak. Corpuscles in the various
stages of division may easily be found. They do not appear to show very
active amoeboid movements in the vessels, though their movements are
sometimes very active when removed from the body.
To recapitulate--some of the cells of the mesoblast of the splanchnopleure
send out processes, these processes unite with the processes from other
cells, and in this way a network is formed. The nuclei of the original
cells divide, and at the points from which the processes start their
division is especially rapid. Some of them acquire especially at these
points a red colour, and so become converted into blood-corpuscles; the
others, together with part of the protoplasm in which they are imbedded,
become converted into an endothelium both for the processes and the masses
of corpuscles; the remaining protoplasm becomes fluid, and thus the
original network of the cells becomes converted into a network of hollow
vessels, filled with fluid, in which corpuscles float.
In reference to the development of the heart, my observations are not quite
complete. It is, however, easy to prove from sections (vide figs. 10 and
11, Pl. 2) that the cavity of the heart is produced by a splitting or
absorption of central cells of the thickened mesoblast of the
splanchnopleure, while its muscular walls are formed from the remaining
cells of this thickened portion. It is produced in the following way:--When
the hypoblast is folded in to form the alimentary canal the mesoblast of
the splanchnopleure follows it closely, and where the splanchnopleure turns
round to assume its normal direction (fig. 11) its mesoblast becomes
thickened. This thickened mass of mesoblast is, as can easily be seen from
figs. 10 and 11, Pl. 2, entirely distinct from the mesoblast which forms
the outside walls of the alimentary canal. At the point where this
thickening occurs an absorption takes place to form the cavity of the
heart. The method in which the cavity is formed can easily be seen from
figs. 10 and 11. It is in fig. 11 shewn as it takes place in the mesoblast
on each side, the folds of the splanchnopleure not having united in the
middle line; and hence a pair of cavities are formed, one on each side. It
is, however, probable that, in the very first formation of the heart, the
cavity is single, being formed after the two ends of the folded mesoblast
have united (vide _hz_, fig. 10). In some cases the two folds of the
mesoblast appear not at first to become completely joined in the middle
line; in this case the cavity of the heart is still complete from side to
side, but the mesoblast-cells which form its muscular walls are deficient
above. By the process of absorption, as I said, a cavity is produced in the
thickened part of the mesoblast of the splanchnopleure, a cavity which is
single in front, but becomes divided further behind, where the folds of the
mesoblast have not united, into two cavities, to form the origin of the
omphalomeseraic veins. As the folding proceeds backwards the starting-point
of the omphalomeseraic veins is also pushed backwards, and the cavities
which were before separated become joined together. From its first
formation the heart is lined internally by an endothelium; this is formed
of flattened cells, spindle-shaped in section. The exact manner of the
origin of this lining I have not been able to determine; it is, however,
probable that some of the central mesoblast-cells are directly converted
into the cells of the endothelium.
I have obtained no evidence enabling me to determine whether Dr Klein is
correct in stating that the cells of the mesoblast in the interior of the
heart become converted partly into blood-corpuscles and partly into a
cellular lining forming the endothelium of the heart, in the same way that
the blood-vessels in the rest of the blastoderm are formed. But I should be
inclined to think that it is very probable--certainly more probable than
that the cavity of the heart is formed by a process of splitting taking
place. Where I have used the word "absorption" in speaking of the formation
of the cavity of the heart, I must be understood as implying that certain
of the interior cells become converted into the endothelium, while others
either form the plasma or become blood-corpuscles.
The originally double formation of the hinder part of the heart probably
explains Dr Afanassiev's statement (_Bullétin de l'Académ. Impériale de St
Petersb._, tom. xiii, pp. 321-335), that he finds the endothelium of the
heart originally dividing its interior into two halves; for when the
partition of the mesoblast which separated at first the two halves of the
heart became absorbed, the endothelium lining of each of the originally
separate vessels would remain complete, dividing the cavity of the heart
into two parts. The partition in the central line is, however, soon
absorbed.
The account given above chiefly differs from that of Remak by not supposing
that the mesoblast-cells which form the heart are in any way split off from
the wall of the alimentary canal.
There can be no doubt that His is wrong in supposing that the heart
originates from the mesoblast of the splanchnopleure and somatopleure
uniting to form its walls, thus leaving a cavity between them in the
centre. The heart is undoubtedly formed out of the mesoblast of the
splanchnopleure only.
Afanassiev's observations are nearer to the truth, but there are some
points in which he has misinterpreted his sections.
Sections Pl. 2, figs. 10 and 11, explain what I have just said about the
origin of the heart. Immediately around the notochord the mesoblast is not
split, but a very little way outside it is seen to be split into two parts
_so_ and _sp_; the former of these follows the epiblast, and together with
it forms the somatopleure, which has hardly begun to be folded at the line
where the sections are taken. The latter (_sp_) forms with the hypoblast
(_hy_) the splanchnopleure, and thus has become folded in to form the walls
of the alimentary canal (_d_). In fig. 11 the folds have not united in the
central line, but in fig. 10 they have so united. In fig. 11, where the
mesoblast, still following the hypoblast, turns back to assume its normal
direction, it is seen to be thickened and to have become split, so that a
cavity (_of_) (of the omphalomeseraic vein) is formed in it on each side,
lined by endothelium.
In the section immediately behind section fig. 11 the mesoblast was
thickened, but had not become split.
In fig. 10 the hypoblast folds are seen to have united in the centre, so as
to form a completely closed digestive canal (_d_); the folds of the
mesoblast have also united, so that there is only a single cavity in the
heart (_hz_), lined, as was the case with the omphalomeseraic veins, by
endothelium.
In conclusion, I have to thank Dr Foster for his assistance and suggestions
throughout the investigations which have formed the subject of these three
short papers, and which were well carried on in the apartments used by him
as a Physiological Laboratory.
EXPLANATION OF PLATE 2.
Fig. 1 is taken from the anterior part of the pellucid area of a thirty
hours' chick, with four protovertebræ. At _n_ is a nucleus with two
nucleoli.
Figs. 2 and 3 are taken from the posterior end of the pellucid area of a
chick with eight protovertebræ. In fig. 3 the nuclei are seen to have
considerably increased in number at the points of starting of the
protoplasmic processes. At _n_ is seen a nucleus with two nucleoli.
Fig. 4 is taken from the anterior part of the pellucid area of an embryo of
thirty-six hours. It shews the narrow processes characteristic of the
anterior part of the pellucid area, and the fewer nuclei. Small spaces,
which have the appearance of vacuoles, are shewn at _v_.
Fig. 5 is taken from the posterior part of the pellucid area of a
thirty-six hours' embryo. It shews the nuclei, with somewhat irregular
nucleoli, which have begun to acquire the red colour of blood-corpuscles;
the protoplasmic processes containing the nuclei; the nuclei in the
protoplasm surrounding the corpuscles, as shewn at _a_, _a´_.
Fig. 6 shews fully formed blood-vessels, in part filled with
blood-corpuscles and in part empty. The walls of the capillaries, formed of
cells, spindle-shaped in section, are shewn, and also the secondary
investment of Klein at _k_, and at _b_ is seen a narrow protoplasmic
process filled with blood-corpuscles.
Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six
hours' embryo. It shews a collection of nuclei which are beginning to
become blood-corpuscles.
Figs. 1-5 are drawn with an 1/8 object-glass. Fig. 6 is on a much smaller
scale. Fig. 7 is intermediate.
Fig. 8.--A transverse section through the dorsal region of a forty-five
hours' embryo; _ao_, aorta with a few blood-corpuscles. v, Blood-vessels,
all of them being formed in the splanchnopleure, and all of them provided
with the secondary investment of Klein; _pe_, pellucid area; _op_, opaque
area.
Fig. 9.--Small portion of a section through the opaque area of a
thirty-five hours' embryo, showing protoplasmic processes, with nuclei
passing from the somatopleure to the splanchnopleure.
Fig. 10.--Section through the heart of a thirty-four hours' embryo. _a_.
Alimentary canal; _hb_, hind brain; _nc_, notochord; _e_, epiblast; _so_,
mesoblast of the somatopleure; _sp_, mesoblast of the splanchnopleure;
_hy_, hypoblast; _hz_, cavity of the heart.
Fig. 11.--Section through the same embryo as fig. 10, and passing through
the orifice of the omphalomeseraic vein. _of_, Omphalomeseraic vein; other
references as above.
These two sections shew that the heart is entirely formed from the
mesoblast of the _splanchnopleure_, and that it is formed by the splitting
of that part of the mesoblast which has turned to assume its normal
direction after being folded in to form the muscular wall of the alimentary
canal. In fig. 11 the cavities so formed on each side have not yet united,
but in fig. 10 they have united. When the folding becomes more complete the
cavities (_of_, _of_) in fig. 11 will unite, and in this way the origin of
the omphalomeseraic veins will be carried further backwards. In the section
immediately behind section 11 the mesoblast had become thickened, but had
not split.
V. A PRELIMINARY ACCOUNT OF THE DEVELOPMENT OF THE ELASMOBRANCH FISHES[10].
Footnote 10: From the _Quarterly Journal of Microscopical
Science_, Vol. XIV. 1874. Read in Section D, at the Meeting of
the British Association at Belfast.
With Plates 3 and 4.
During the spring of the present year I was studying at the Zoological
Station, founded by Dr Dohrn at Naples, and entirely through its agency was
supplied with several hundred eggs of various species of Dog-fish
(Selachii)--a far larger number than any naturalist has previously had an
opportunity of studying. The majority of the eggs belonged to an oviparous
species of _Mustelus_, but in addition to these I had a considerable number
of eggs of two or three species of _Scyllium_, and some of the Torpedo.
Moreover, since my return to England, Professor Huxley has most liberally
given me several embryos of _Scyllium stellare_ in a more advanced
condition than I ever had at Naples, which have enabled me to fill up some
lacunæ in my observations.
On many points my investigations are not yet finished, but I have already
made out a number of facts which I venture to believe will add to our
knowledge of vertebrate embryology; and since it is probable that some time
will elapse before I am able to give a complete account of my
investigations, I have thought it worth while preparing a preliminary paper
in which I have briefly, but I hope in an intelligible manner, described
some of the more interesting points in the development of the
Elasmobranchii. The first-named species (_Mustelus_ sp.?) was alone used
for the early stages, for the later ones I have also employed the other
species, whose eggs I have had; but as far as I have seen at present, the
differences between the various species in early embryonic life are of no
importance.
Without further preface I will pass on to my investigations.
_The Egg-shell._
In the eggs of all the species of Dog-fishes which I have examined the yolk
lies nearest that end of the quadrilateral shell which has the shortest
pair of strings for attachment. This is probably due to the shape of the
cavity of the shell, and is certainly not due to the presence of any
structures similar to chalazæ.
_The Yolk._
The yolk is not enclosed in any membrane comparable to the vitelline
membrane of Birds, but lies freely in a viscid albumen which fills up the
egg-capsule. It possesses considerable consistency, so that it can be
removed into a basin, in spite of the absence of a vitelline membrane,
without falling to pieces. This consistency is not merely a property of the
yolk-sphere as a whole, but is shared by every individual part of it.
With the exception of some finely granular matter around the blastoderm,
the yolk consists of rather small, elliptical, highly refracting bodies,
whose shape is very characteristic and renders them easily recognizable. A
number of striæ like those of muscle are generally visible on most of the
spherules, which give them the appearance of being in the act of breaking
up into a series of discs; but whether these striæ are normal, or produced
by the action of water I have not determined.
_Position of the Blastoderm._
The blastoderm is always situated, immediately after impregnation, near the
pole of the yolk which lies close to the end of the egg-capsule. Its
position varies a little in the different species and is not quite constant
in different eggs of the same species. But this general situation is quite
invariable. It is of about the same proportional size as the blastoderm of
a bird.
_Segmentation._
In a fresh specimen, in which segmentation has only just commenced, the
blastoderm or germinal disc appears as a circular disc, distinctly marked
off by a dark line from the rest of the yolk. This line, as is proved by
sections, is the indication of a very shallow groove. The appearance of
sharpness of distinction between the germ and the yolk is further
intensified by their marked difference of colour, the germ itself being
usually of a darker shade than the remainder of the yolk; while around its
edge, and apparently sharply separated from it by the groove before
mentioned, is a ring of a different shade which graduates at its outer
border into the normal shade of the yolk.
These appearances are proved by transverse sections to be deceptive. There
is no sharp line either at the sides or below separating the blastoderm
from the yolk. In the passage between the fine granular matter of the germ
to the coarser yolk-spheres every intermediate size of granule is present;
and, though the space between the two is rather narrow, in no sense of the
word can there be said to be any break or line between them.
This gradual passage stands in marked contrast with what we shall find to
be the case at the close of the segmentation. In the youngest egg which I
had, the germinal disc was already divided into four segments by two
furrows at right angles. These furrows, however, did not reach its edge;
and from my sections I have found that they were not cut off below by any
horizontal furrow. So that the four segments were continuous below with the
remainder of the germ without a break.
In the next youngest specimen which I had, there were already present
eighteen segments, somewhat irregular in size, but which might roughly be
divided into an outer ring of larger spheres, separated, as it were, by a
circular furrow from an inner series of smaller segments. The furrows in
this case reached quite to the edge of the germinal disc.
The remarks I made in reference to the earlier specimen about the
separation of the germ from the yolk apply in every particular to the
present one. The external limit of the blastoderm was not defined by a true
furrow, and the segmentation furrows still ended below without meeting any
horizontal furrows, so that the blastoderm was not yet separated by any
line from the remainder of the yolk, and the segments of which it was
composed were still only circumscribed upon five sides. In this particular
the segmentation in these animals differs materially from that in the Bird,
where the horizontal furrows appear very early.
In each segment a nucleus was generally to be seen in sections. I will,
however, reserve my remarks upon the nature of the nuclei till I discuss
the nuclei of the blastoderm as a whole.
For some little time the peripheral segments continue larger than the more
central ones, but this difference of size becomes less and less marked, and
before the segments have become too small to be seen with the simple
microscope, their size appears to be uniform over the whole surface of the
blastoderm.
In the blastoderms somewhat older than the one last described the segments
have already become completely separate masses, and each of them already
possesses a distinct nucleus. They form a layer one or two segments deep.
The limits of the blastoderm are not, however, defined by the already
completed segments, but outside these new segments continue to be formed
around nuclei which appear in the yolk. At this stage there is, therefore,
no line of demarcation between the germ and the yolk, but the yolk is being
bored into, so to speak, by a continuous process of fresh segmentation.
The further segmentation of the already existing spheres, and the formation
of new ones from the yolk below and to the sides, continues till the
central cells acquire their final size, the peripheral ones being still
large, and undefined towards the yolk. These also soon reach the final
size, and the blastoderm then becomes rounded off towards the yolk and
sharply separated from it.
_The Nuclei of the Yolk._
Intimately connected with the segmentation is the appearance and history of
a number of nuclei which arise in the yolk surrounding the blastoderm.
When the horizontal furrows appear which first separate the blastoderm from
the yolk, the separation does not occur along the line of passage from the
fine to the coarse yolk, but in the former at some distance from this line.
The blastoderm thus rests upon a mass of finely granular material, from
which, however, it is sharply separated. At this time there appear in this
finely granular material a number of nuclei of a rather peculiar character.
They vary immensely in size--from that of an ordinary nucleus to a size
greater than the largest blastoderm-cell.
In Pl. 3, fig. 1, _n_, is shewn their distribution in this finely granular
matter and their variation in size. But whatever may be their size, they
always possess the same characteristic structure. This is shewn in Pl. 3,
figs. 1 and 2, _n_.
They are rather irregular in shape, with a tendency when small to be
roundish, and are divided by a number of lines into distinct areas, in each
of which a nucleolus is to be seen. The lines dividing them into these
areas have a tendency (in the smaller specimens) to radiate from the
centre, as shewn in Pl. 3, fig. 1.
These nuclei colour red with hematoxylin and carmine and brown with osmic
acid, while the nucleoli or granules contained in the areas also colour
_very intensely_ with all the three above-named reagents.
With such a peculiar structure, in favourable specimens these nuclei are
very easily recognised, and their distribution can be determined without
difficulty. They are not present alone in the finely granular yolk, but
also in the coarsely granular yolk adjoining it. They form very often a
special row, sometimes still more markedly than in Pl. 3, fig. 1, along the
floor of the segmentation cavity. They are not, however, found alone in the
yolk. All the blastoderm-cells in the earlier stages possess precisely
similar nuclei! From the appearance of the first nucleus in a
segmentation-sphere till a comparatively late period in development, every
nucleus which can be distinctly seen is found to be of this character. In
Pl. 3, fig. 2, this is very distinctly shewn.
(1) We have, then, nuclei of this very peculiar character scattered through
the sub-germinal granular matter, and also universally present in the cells
of the blastoderm. (2) These nuclei are distributed in a special manner
under the floor of the segmentation cavity on which new cells are
continually appearing. Putting these two facts together, there would be the
strongest presumption that these nuclei do actually become the nuclei of
cells which enter the blastoderm, and such is actually the case. In my
account of the segmentation I have, indeed, already mentioned this, and I
will return to it, but before doing so will enter more fully into the
distribution of these nuclei in the yolk.
They appear in small numbers around the blastoderm at the close of
segmentation, and round each one of them there may at this time be seen in
osmic acid specimens, and with high powers, a fine network similar to but
finer than that represented in Pl. 3, fig. 2. This network cannot, as a
general rule, be traced far into the yolk, but in some exceptionally thin
specimens it may be seen in any part of the fine granular yolk around the
blastoderm, the meshes of the network being, however, considerably coarser
between than around the nuclei. This network may be seen in the fine
granular material around the germ till the latest period of which I have
yet cut sections of the blastoderm. In the later specimens, indeed, it is
very much more distinctly seen than in the earlier, owing to the fact that
in parts of the blastoderm, especially under the embryo, the yolk-granules
have disappeared partly or entirely, leaving only this fine network with
the nuclei in it.
A specimen of this kind is represented in Pl. 3, fig. 2, where the meshes
of the network are seen to be finer immediately around the nuclei, and
coarser in the intervals. The specimen further shows in the clearest manner
that this network is not divided into areas, each representing a cell and
each containing a nucleus. I do not know to what extent this network
extends into the yolk. I have never yet seen the limits of it, though it is
very common to see the coarsest yolk-granules lying in its meshes. Some of
these are shewn in Pl. 3, fig. 2, _yk_.
This network of lines[11] (probably bubbles) is characteristic of many
cells, especially ova. We are, therefore, forced to believe that the fine
granular and probably coarser granular yolk of this meroblastic egg
consists of an active organized basis with passive yolk-spheres imbedded in
it. The organized basis is especially concentrated at the germinal pole of
the egg, but becomes less and less in quantity, as compared with the
yolk-spheres, the further we depart from this.
Footnote 11: The interpretation of this network is entirely
due to Dr Kleinenberg, who suggested it to me on my shewing him
a number of specimens exhibiting the nuclei and network.
Admitting, as I think it is necessary to do, the organized condition of the
whole yolk-sphere, there are two possible views as to its nature. We may
either take the view that it is one gigantic cell, the ovum, which has
grown at the expense of the other cells of the egg-follicle, and that these
cells in becoming absorbed have completely lost their individuality; or we
may look upon the true formative yolk (as far as we can separate it from
the remainder of the food-yolk) as the remains of one cell (the primitive
ovum), and the remainder of the yolk as a body formed from the coalescence
of the other cells of the egg-follicle, which is adherent to, but has not
coalesced with, the primitive ovum, the cells in this case not having
completely lost their individuality; and to these cells, the nuclei, I have
found, must be supposed to belong.
The former view I think, for many reasons, the most probable. The share of
these nuclei in the segmentation, and the presence of similar nuclei in the
cells of the germ, both support it, and are at the same time difficulties
in the way of the other view. Leaving this question which cannot be
discussed fully in a preliminary paper like the present one, I will pass on
to another important question, viz.:
How do these nuclei originate? Are they formed by the division of the
pre-existing nuclei, or by an independent formation? It must be admitted
that many specimens are strongly in favour of the view that they increase
by division. In the first place, they are often seen "two together;"
examples of this will be seen in Pl. 3, fig. 1. In the second place, I have
found several specimens in which five or six appear close together, which
look very much as if there had been an actual division into six nuclei. It
is, however, possible in this case that the nuclei are really connected
below and only appear separate, owing to the crenate form of the mass.
Against this may be put the fact that the division of a nucleus is by no
means so common as has been sometimes supposed, that in segmentation it has
very rarely been observed that the nucleus of a sphere first divides[12],
and that then segmentation takes place, but segmentation generally occurs
and then a new nucleus arises in each of the newly formed spheres. Such
nuclei as I have described are rare; they have, however, been observed in
the egg of a _Nephelis_ (one of the Leeches), and have in that case been
said to divide. Dr Kleinenberg, however, by following a single egg through
the whole course of its development, has satisfied himself that this is not
the case, and that, further, these nuclei in Nephelis never form the nuclei
of newly developing cells.
Footnote 12: Kowalevsky ("Beiträge zur Entwicklungsgeschichte
der Holothurien," _Mémoires de l'Ac. Imp. de St Petersbourg_,
vii ser., Vol. XI. 1867) describes the division of nuclei
during segmentation in the Holothurians, and other observers
have described it elsewhere.
I must leave it an open question, and indeed one which can hardly be solved
from sections, whether these nuclei arise freely or increase by division,
but I am inclined to believe that both processes may possibly take place.
In any case their division does not appear to determine the segmentation or
segregation of the protoplasm around them.
As was mentioned in my account of the segmentation, these nuclei first
appear during that process, and become the nuclei of the freshly formed
segmentation spheres. At the close of segmentation a few of them are still
to be seen around the blastoderm, but they are not very numerous.
From this period they rapidly increase in number, up to the commencement of
the formation of the embryo as a body distinct from the germ. Though before
this period they probably become the nuclei of veritable cells which enter
the germ, it is not till this period, when the growth of the blastoderm
becomes very rapid and it commences to spread over the yolk, that these new
cells are formed in large numbers. I have many specimens of this age which
shew the formation of these new cells with great clearness. This is most
distinctly to be seen immediately below the embryo, where the
yolk-spherules are few in number. At the opposite end of the blastoderm I
believe that more of these cells are formed, but, owing to the presence of
numerous yolk-spherules, it is much more difficult to make certain of this.
As to the final destination of these cells, my observations are not yet
completed. Probably a large number of them are concerned in the formation
of the vascular system, but I will give reasons later on for believing that
some of them are concerned in the formation of the walls of the digestive
canal and of other parts.
I will conclude my account of these nuclei by briefly summarizing the
points I have arrived at in reference to them.
A portion, or more probably the whole, of the _yolk_ of the Dog-fish
consists of _organized material_, in which nuclei appear and increase
either by _division_ or by a process of _independent_ formation, and a
great number of these subsequently become the nuclei of cells formed around
them, frequently at a distance from the germ, which then travel up and
enter it.
The formation of cells in the yolk, apart from the general process of
segmentation, has been recognised by many observers. Kupffer (_Archiv. für
Micr. Anat._, Bd. IV. 1868) and Owsjannikow ("Entwicklung der Coregonus,"
_Bulletin der Akad. St Petersburgh_, Vol. XIX.) in osseous fishes[13], Ray
Lankester (_Annals and Mag. of Nat. Hist._ Vol. XI. 1873, p. 81) in
Cephalopoda, Götte (_Archiv. für Micr. Anat._ Vol. X.) in the chick, have
all described a new formation of cells from the so-called food-yolk. The
organized nature of the whole or part of this, previous to the formation of
the cells from it, has not, however, as a rule, been distinctly recognised.
In the majority of cases, as, for instance, in Loligo, the nucleus is not
the first thing to be formed, but a plastide is first formed, in which a
nucleus subsequently makes its appearance.
Footnote 13: Götte, at the end of a paper on "The Development
of the Layers in the Chick" (_Archiv. für Micr. Anat._, Vol. X.
1873, p. 196), mentions that the so-called cells in Osseous
fishes which Oellacher states to have migrated into the yolk,
and which are clearly the same as those mentioned by
Owsjannikow, are really not _cells_, but large _nuclei_. If
this statement is correct the phenomena in Osseous fishes are
precisely the same as those I have described in the Dog-fish.
_Formation of the Layers._
Leaving these nuclei, I will now pass on to the formation of the layers.
At the close of segmentation the surface of the blastoderm is composed of
cells of a uniform size, which, however, are too small to be seen by the
aid of the simple microscope.
The cells of this uppermost layer are somewhat columnar, and can be
distinguished from the remainder of the cells of the blastoderm as a
separate layer. This layer forms the epiblast; and the Dog-fish agree with
Birds, Batrachians, and Osseous fish in the very early differentiation of
it.
The remainder of the cells of the blastoderm form a mass, many cells deep,
in which it is impossible as yet or till a very considerably later period
to distinguish two layers. They may be called the _lower layer cells_. Some
of them near the edge of this mass are still considerably larger than the
rest, but they are, as a whole, of a fairly uniform size. Their nuclei are
of the same character as the nuclei in the yolk.
There is one point to be noticed in the shape of the blastoderm as a whole.
It is unsymmetrical, and a much larger number of its cells are found
collected at one end than at the other. This absence of symmetry is found
in all sections which are cut parallel to the long axis of the egg-capsule.
The thicker end is the region where the embryo will subsequently appear.
This very early appearance of distinction in the blastoderm between the end
at which the embryo will appear, and the non-embryonic end is important,
especially as it shews the affinity of the modes of development of Osseous
fishes and the Elasmobranchii. Oellacher (_Zeitschrift für Wiss. Zoologie_,
Vol. XXXIII. 1873) has shewn, and, though differing from him on many other
points, on this point Götte (_Arch. für Micr. Anat._ Vol. IX. 1873) agrees
with him, that a similar absence of symmetry by which the embryonic end of
the germ is marked off, occurs almost immediately after the end of
segmentation in Osseous fishes. In the early stages of development there
are a number of remarkable points of agreement between the Osseous fish and
the Dog-fish, combined with a number of equally remarkable points of
difference. Some of these I shall point out as I proceed with my
description.
The embryonic end of the germ is always the one which points towards the
pole of the yolk farthest removed from the egg-capsule.
The germ grows, but not very rapidly, and without otherwise undergoing any
very appreciable change, for some time.
The growth at these early periods appears to be particularly slow,
especially when compared with the rapid manner in which some of the later
stages of the development are passed through.
The next important change which occurs is the formation of the so-called
"segmentation cavity."
This forms a very marked feature throughout the early stages. It appears,
however, to have somewhat different relations to the blastoderm than the
homologous structure in other vertebrates. In its earliest stage which I
have observed, it appears as a small cavity in the centre of the lower
layer cells. This grows rapidly, and its roof becomes composed of epiblast
and only a thin lining of "_lower layer_" cells, while its floor is formed
by the yolk (Pl. 3, fig. 3, _sg_). In the next and third stage (Pl. 3,
fig. 4, _sg_) its floor is formed by a thin layer of cells, its roof
remaining as before. It has, however, become a less conspicuous formation
than it was; and in the last (fourth) stage in which it can be
distinguished it is very inconspicuous, and almost filled up by cells.
What I have called the second stage corresponds to a period in which no
trace of the embryo is to be seen. In the third stage the embryonic end of
the blastoderm projects outwards to form a structure which I shall speak of
as the "embryonic rim," and in the fourth and last stage a distinct
medullary groove is formed. For a considerable period during the second
stage the segmentation cavity remains of about the same size; during the
third stage it begins to be encroached upon, and becomes smaller both
absolutely, and relatively to the increased size of the germ.
The segmentation cavity of the Dog-fish most nearly agrees with that of
Osseous fishes in its mode of formation and relation to the embryo.
Dog-fish resemble Osseous fish in the fact that their embryos are entirely
formed from a portion of the germ which does not form part of the roof of
the segmentation cavity, so that the cells forming the roof of the
segmentation cavity take _no share_ at any time in the formation of their
embryos. They further agree with Osseous fish (always supposing that the
descriptions of Oellacher, _loc. cit._, and Götte, _Archiv. für Micr.
Anat._ Bd. IX. are correct) in the floor of the segmentation cavity being
formed at one period by yolk. Together with these points of similarity
there are some important differences.
(1) The segmentation cavity in the Osseous fish from the first arises as a
cavity between the yolk and the blastoderm, and its floor is never at any
period covered with cells. In the Dog-fish, as we have said above, both in
the earlier and later periods the floor is covered with cells.
(2) The roof in the Dog-fish is _invariably_ formed by the epiblast and a
row of flattened lower layer cells.
According to both Götte and Oellacher the roof of the segmentation cavity
in Osseous fishes is in the earlier stages formed _alone_ of the two layers
which correspond with the single layer forming the epiblast in the
Dog-fish. In Osseous fishes it is very difficult to distinguish the various
layers, owing to the similarity of their component cells. In Dog-fish this
is very easy, owing to the great distinctness of the epiblast, and it
appears to me, on this account, very probable that the two above-named
observers may be in error as to the constitution of its roof in the Osseous
fish. With both the Bird and the Frog the segmentation cavity of the
Dog-fish has some points of agreement, and some points of difference, but
it would take me too far from my present subject to discuss them.
When the segmentation cavity is first formed, no great changes have taken
place in the cells forming the blastoderm. The upper layer--the
_epiblast_--is composed of a single layer of columnar cells, and the
remainder of the cells of blastoderm, forming the lower layer, are of a
fairly uniform size, and polygonal from mutual pressure. The whole edge of
the blastoderm is thickened, but this thickening is especially marked at
its embryonic end.
This thickened edge of the blastoderm is still more conspicuous in the next
and second stage (Pl. 3, fig. 3).
In the second stage the chief points of progress, in addition to the
increased thickness of the edge of the blastoderm, are--
(1) The increased thickness and distinctness of the epiblast, caused by its
cells becoming more columnar, though it remains as a one-cell-thick layer.
(2) The disappearance of the cells from the floor of the segmentation
cavity.
The lower layer cells have undergone no important changes, and the
blastoderm has increased very little if at all in size.
From Pl. 3, fig. 3, it is seen that there is a far larger collection of
cells at the embryonic than at the opposite end.
Passing over some rather unimportant stages, I will come to the next
important one.
The general features of this (the third) stage in a surface view are--
(1) The increase in size of the blastoderm.
(2) The diminution in size of the segmentation cavity, both relatively and
absolutely.
(3) The appearance of a portion of the blastoderm projecting beyond the
rest over the yolk. This projecting rim extends for nearly half the
circumference of the yolk, but is most marked at the point where the embryo
will shortly appear. I will call it the "embryonic rim."
These points are still better seen from sections than from surface views,
and will be gathered at once from an inspection of Pl. 3, fig. 4.
The epiblast has become still more columnar, and is markedly thicker in the
region where the embryo will appear. But its most remarkable feature is
that at the outer edge of the "embryonic rim" (_er_) it turns round and
becomes continuous with the lower layer cells. This feature is most
important, and involves some peculiar modifications in the development. I
will, however, reserve a discussion of its meaning till the next stage.
The only other important feature of this stage is the appearance of a layer
of cells on the floor of the segmentation cavity.
Does this layer come from an ingrowth from the thickened edge of the
blastoderm, or does it arise from the formation of new cells in the yolk?
It is almost impossible to answer this question with certainty. The
following facts, however, make me believe that the newly formed cells do
play an important part in the formation of this layer.
(1) The presence at an earlier date of almost a row of nuclei under the
floor of the segmentation cavity (Pl. 3, fig. 1).
(2) The presence on the floor of the cavity of such large cells as those
represented in fig. 1, _bd_, cells which are very different, as far as the
size and granules are concerned, from the remainder of the cells of the
blastoderm.
On the other hand, from this as well as other sections, I have satisfied
myself that there is a distinct ingrowth of cells from the embryonic
swelling. It is therefore most probable that both these processes, viz. a
fresh formation and an ingrowth, have a share in the formation of the layer
of cells on the floor of the segmentation cavity.
In the next stage we find the embryo rising up as a distinct body from the
blastoderm, and I shall in future speak of the body, which now becomes
distinct as the embryo. It corresponds with what Kupffer (_loc. cit._) in
his paper on the "Osseous Fishes" has called the "embryonic keel." This
starting-point for speaking of the embryo as a distinct body is purely
arbitrary and one merely of convenience. If I wished to fix more correctly
upon a period which could be spoken of as marking the commencing formation
of the embryo, I should select the time when structures first appear to
mark out the portion of the germ from which the embryo becomes formed; this
period would be in the Elasmobranchii, as in the Osseous fish, at the
termination of segmentation, when the want of symmetry between the
embryonic end of the germ and the opposite end first appears.
I described in the last stage the appearance of the "embryonic rim." It is
in the middle point of this, where it projects most, that the formation of
the embryo takes place. There appear two parallel folds extending from the
edge of the blastoderm towards the centre, and cut off at their central end
by another transverse fold. These three folds raise up, between them, a
flat broadish ridge, "_the embryo_" (Pl. 3, fig. 5). The head end of the
embryo is the end nearest the centre of the blastoderm, the tail end being
the one formed by its (the blastoderm's) edge.
Almost from its first appearance this ridge acquires a shallow groove--the
medullary groove (Pl. 3, fig. 5, _mg_)--along its middle line, where the
epiblast and hypoblast are in absolute contact (vide fig. 6_a_, 7_a_, 7_b_,
&c.) and where the mesoblast (which is already formed by this stage) is
totally absent. This groove ends abruptly a little before the front end of
the embryo, and is deepest in the middle and wide and shallow behind.
On each side of it is a plate of mesoblast equivalent to the combined
vertebral and lateral plates of the Chick. These, though they cannot be
considered as entirely the cause of the medullary groove, may perhaps help
to make it deeper. In the parts of the germ outside the embryo the
mesoblast is again totally absent, or, more correctly, we might say that
outside the embryo the _lower layer cells_ do not become differentiated
into hypoblast and mesoblast, and remain continuous only with the lower of
the two layers into which the _lower layer cells_ become differentiated in
the body of embryo. This state of things is not really very different from
what we find in the Chick. Here outside the embryo (_i.e._ in the opaque
area) there is a layer of cells in which no differentiation into hypoblast
and mesoblast takes place, but the layer remains continuous rather with the
hypoblast than the mesoblast.
There is one peculiarity in the formation of the mesoblast which I wish to
call attention to, _i.e._ its formation as two lateral masses, one on each
side of the middle line, but not continuous across this line (vide figs.
6_a_ and 6_b_, and 7_a_ and 7_b_). Whether this remarkable condition is the
most primitive, _i.e._ whether, when in the stage before this the mesoblast
is first formed, it is only on each side of the middle line that the
differentiation of the lower layer cells into hypoblast and mesoblast takes
place, I do not certainly know, but it is undoubtedly a very early
condition of the mesoblast. The condition of the mesoblast as two plates,
one on each side of the neural canal, is precisely similar to its embryonic
condition in many of the Vermes, _e.g. Euaxes_ and _Lumbricus_. In these
there are two plates of mesoblast, one on each side of the nervous cord,
which are known as the _Germinal streaks_ (Keimstreifen) (vide Kowalevsky
"Würmern u. Arthropoden"; _Mém. de l'Acad. Imp. St Petersbourg_, 1871).
From longitudinal sections I have found that the segmentation cavity has
ceased by this stage to have any distinct existence, but that the whole
space between the epiblast and the yolk is filled up with a mass of
elongated cells, which probably are solely concerned in the formation of
the vascular system. The thickened posterior edge of the blastoderm is
still visible.
At the embryonic end of the blastoderm, as I pointed out in an earlier
stage, the epiblast and the lower layer cells are perfectly continuous.
Where they join the epiblast, the _lower layer cells_ become distinctly
divided, and this division commenced even in the earlier stage, into two
layers; a lower one, more directly continuous with the epiblast, consisting
of cells somewhat resembling the epiblast-cells, and an upper one of more
flattened cells (Pl. 3, fig. 4, _m_). The first of these forms the
hypoblast, and the latter the mesoblast. They are indicated by _hy_ and _m_
in the figures. The hypoblast, as I said before, remains continuous with
the whole of the rest of lower layer cells of the blastoderm (vide
fig. 7_b_). This division into hypoblast and mesoblast commences at the
earlier stage, but becomes much more marked during this one.
In describing the formation of the hypoblast and mesoblast in this way I
have assumed that they are formed out of the large mass of lower layer
cells which underlie the epiblast at the embryonic end of the blastoderm.
But there is another and, in some ways, rather a tempting view, viz. to
suppose that the epiblast, where it becomes continuous with the hypoblast,
in reality becomes involuted, and that from this involuted epiblast are
formed the whole mesoblast and hypoblast.
In this case we would be compelled to suppose that the mass of lower layer
cells which forms the embryonic swelling is used as food for the growth of
the involuted epiblast, or else employed solely in the growth over the yolk
of the non-embryonic portion of the blastoderm; but the latter possibility
does not seem compatible with my sections.
I do not believe that it is possible, from the examination of sections
alone, to decide which of these two views (viz. whether the epiblast is
involuted, or whether it becomes merely continuous with the lower layer
cells) is the true one. The question must be decided from other
considerations.
The following ones have induced me to take the view that there is no
involution, but that the mesoblast and hypoblast are formed from the lower
layer cells.
(1) That it would be rather surprising to find the mass of lower layer
cells which forms the "embryo swelling" playing no part in the formation of
embryo.
(2) That the view that it is the lower layer cells from which the hypoblast
and mesoblast are derived agrees with the mode of formation of these two
layers in the Bird, and also in the Frog; since although, in the latter
animal, there is an involution, this is not of the epiblast, but of the
larger cells of the lower pole of the yolk, which in part correspond with
what I have called the lower layer cells in the Dog-fish.
If the view be accepted that it is from the lower layer cells that the
hypoblast and mesoblast are formed, it becomes necessary to explain what
the continuity of the hypoblast with the epiblast means.
The explanation of this is, I believe, the keystone to the whole position.
The vertebrates may be divided as to their early development into two
classes, viz. those with _holoblastic ova_, in which the digestive canal is
formed by an _involution_ with the presence of an "_anus of Rusconi_."
This class includes "Amphioxus," the "Lamprey," the "Sturgeon," and
"Batrachians."
The second class are those with _meroblastic ova_ and no _anus of Rusconi_,
and with an alimentary canal formed by the infolding of the sheet of
hypoblast, the digestive canal remaining in communication with the
food-yolk for the greater part of embryonic life by an umbilical canal.
This class includes the "Elasmobranchii," "Osseous fish," "Reptiles," and
"Aves."
The mode of formation of the alimentary canal in the first class is clearly
the more primitive; and it is equally clear that its mode of formation in
the second class is an adaptation due to the presence of the large quantity
of food-yolk.
In the Dog-fish I believe that we can see, to a certain extent, how the
change from the one to the other of these modes of development of the
alimentary canal took place.
In all the members of the first class, viz. "_Amphioxus_," the "Lamprey,"
the "Sturgeon," and the "Batrachians," the epiblast becomes continuous with
the hypoblast at the so-called "anus of Rusconi," and the alimentary canal,
potentially in all and actually in the Sturgeon (vide Kowalevsky,
Owsjannikow, and Wagner, _Bulletin der Acad. d. St Petersbourg_, Vol. XIV.
1870, "Entwicklung der Störe"), communicates freely at its extreme hind end
with the neural canal. The same is the case in the Dog-fish. In these, when
the folding in to form the alimentary canal on the one hand, and the neural
on the other, takes place, the two foldings unite at the corner, where the
epiblast and hypoblast are in continuity, and place the two tubes, the
neural and alimentary, in free communication with each other[14].
Footnote 14: This has been already made out by Kowalevsky,
"Würmern u. Arthropoden," _loc. cit._
There is, however, nothing corresponding with the "anus of Rusconi," which
merely indicates the position of the involution of the digestive canal, and
subsequently completely closes up, though it nearly coincides in position
with the true anus in the Batrachians, &c.
This remarkable point of similarity between the Dog-fish's development and
the normal mode of development in the first class (the holoblastic) of
vertebrates, renders it quite clear that the continuity of the epiblast and
hypoblast in the Dogfish is really the remnant of a more primitive
condition, when the alimentary canal was formed by an involution. Besides
the continuity between neural and alimentary canals, we have other remnants
of the primitive involution. Amongst these the most marked is the formation
of the embryonic rim, which is nothing less than the commencement of an
involution. Its form is due to the flattened, sheet-like condition of the
germ. In the mode in which the alimentary canal is closed in front I shall
shew there are indications of the primitive mode of formation of the
alimentary canal; and in certain peculiarities of the anus, which I shall
speak of later, we have indications of the primitive anus of Rusconi; and
finally, in the general growth of the epiblast (small cells of the upper
pole of the Batrachian egg) over the yolk (lower pole of the Batrachian
egg), we have an example of the manner in which the primitive involution,
to form the alimentary canal, invariably disappears when the quantity of
yolk in an egg becomes very great.
I believe that in the Dog-fish we have before our eyes one of the steps by
which a direct mode of formation comes to be substituted for an _in_direct
one by involution. We find, in fact, in the Dog-fish, that the cells from
which are derived the mesoblast and hypoblast come to occupy their final
position in the primitive arrangement of the cells during segmentation, and
not by a subsequent and secondary involution.
This change in the mode of formation of the alimentary canal is clearly a
result of change of mechanical conditions from the presence of the large
food-yolk.
Excellent parallels to it will be found amongst the Mollusca. In this class
the presence or absence of food-yolk produces not very dissimilar changes
to those which are produced amongst vertebrates from the same cause.
The continuity of the hypoblast and epiblast at the embryonic rim is a
remnant which, having no meaning or function, except in reference to the
earlier mode of development, is likely to become lost, and in Birds no
trace of it is any longer to be found.
I will not in the present preliminary paper attempt hypothetically to trace
the steps by which the involution gradually disappeared, though I do not
think it would be very difficult to do so. Nor will I attempt to discuss
the question whether the condition with a large amount of food-yolk (as
seems more probable) was twice acquired--once by the Elasmobranchii and
Osseous fishes, and once by Reptiles and Birds--or whether only once, the
Reptiles and Birds being lineal descendants of the Dog-fish.
In reference to the former point, however, I may mention that the
Batrachians and Lampreys are to a certain extent intermediate in condition
between the _Amphioxus_ and the Dog-fishes, since in them the yolk becomes
divided during segmentation into lower layer cells and epiblast, but a
modified involution is still retained, while the Dog-fish may be looked
upon as intermediate between Birds and Batrachians, the continuity at the
hind end between the epiblast and hypoblast being retained by them, though
not the involution.
It may be convenient here to call attention to some of the similarities and
some of the differences which I have not yet spoken of between the
development of Osseous fish and the Dog-fish in the early stages. The
points of similarity are--(1) The swollen edge of the blastoderm. (2) The
embryo-swelling. (3) The embryo-keel. (4) The spreading of the blastoderm
over the yolk-sac from a point corresponding with the position of the
embryo, and not with the centre of the germ. The growth is almost nothing
at that point, and most rapid at the opposite pole of the blastoderm, being
less and less rapid along points of the circumference in proportion to
their proximity to the embryonic swelling. (5) The medullary groove.
In external appearance the early embryos of Dog-fish and Teleostei are very
similar; some of my drawings could almost be substituted for those given by
Oellacher. This similarity is especially marked at the first appearance of
the medullary groove. In the Dog-fish the medullary groove becomes
converted into the medullary canal in the same way as in Birds and all
other vertebrates, except Osseous fishes, where it comes to nothing, and
is, in fact, a rudimentary structure. But in spite of Oellacher's
assertions to the contrary, I am convinced from the similarity of its
position and appearance to the true medullary groove in the Dog-fish, that
the groove which appears in Osseous fishes is the true medullary groove;
although Oellacher and Kuppfer appear to have conclusively proved that it
does not become converted into the medullary canal. The chief difference
between the Dog-fish and Osseous fish, in addition to the point of
difference about the medullary groove, is that the epiblast is in the
Dog-fish a single layer, and not divided into nervous and epidermic layers
as in Osseous fish, and this difference is the more important, since,
throughout the whole period of development till after the commencement of
the formation of the neural canal, the epiblast remains in Dog-fish as a
one-cell-deep layer of cells, and thus the possibility is excluded of any
concealed division into a neural and epidermic layer, as has been supposed
to be the case by Stricker and others in Birds.
_Development of the Embryo._
After the embryo has become definitely established, for some time it grows
rapidly in length, without externally undergoing other important changes,
with the exception of the appearance of two swellings, one on each side of
its tail.
These swellings, which I will call the _Caudal lobes_ (figs. 8 and 9,
_ts_), are also found in Osseous fishes, and have been called by Oellacher
the _Embryonal saum_. They are caused by a thickening of mesoblast on each
side of the hind end of the embryo, at the edge of the embryonic rim, and
form a very conspicuous feature throughout the early stages of the
development of the Dog-fish, and are still more marked in the Torpedo
(Pl. 3, fig. 9). Although from the surface the other changes which are
visible are very insignificant, sections shew that the _notochord_ is
commencing to be formed.
I pointed out that beneath the medullary groove the epiblast and hypoblast
were not separated by any interposed mesoblast. Along the line (where the
mesoblast is deficient) which forms the long axis of the embryo, a rod-like
thickening of the hypoblast appears (Pl. 3, figs. 7_a_ and 7_b_, _ch_ and
_ch´_), first at the head end of the embryo, and gradually extends
backwards. This is the rudiment of the notochord; it remains attached for
some time to the hypoblast, and becomes separated from it first at the head
end of the embryo, and the separation is then carried backwards. This
thickening of the hypoblast projects up and comes in contact with the
epiblast, and in the later stages with bad (especially chromic-acid)
specimens the line of separation between the epiblast and the thickening
may become a little obscured, and might possibly lead to the supposition
that a structure similar to that which has been called the "_axis cord_"
was present. In all my best (osmic-acid) specimens the line of junction is
quite clear; and any one who is aware how easily two separate masses of
cells may be made indistinguishably to fuse together from simple pressure
will not be surprised to find the occasional obscurity of the line of
junction between the epiblast and hypoblast. In the earlier stage of the
thickening there is never in the osmic-acid preparations any appearance of
fusion except in very badly prepared ones. Its mode of formation will be
quite clear without further description from an inspection of Pl. 3, figs.
7_a_ and 7_b_, _ch_ and _ch´_. Both are taken from one embryo. In
fig. 7_b_, the most anterior of the two, the notochord has become quite
separated from the hypoblast. In fig. 7_a_, _ch_, there is only a very
marked thickening of hypoblast, which reaches up to the epiblast, but the
thickening is still attached to the hypoblast. Had I had space to insert a
drawing of a third section of the same embryo there would only have been a
slight thickening of the hypoblast. In the earlier stage it will be seen,
by referring to figs. 6_a_ and 6_b_, that there is no sign of a thickening
of the hypoblast. My numerous sections (all made from embryos hardened in
osmic acid) shewing these points are so clear that I do not think there can
be any doubt whatever of the notochord being formed as a thickening of the
hypoblast. Two interpretations of this seem possible.
I mentioned that the mesoblast appeared to be primitively formed as two
independent sheets, _split off, so to speak, from the hypoblast_, one on
each side of the middle line of the embryo. If we looked upon the notochord
as a third _median sheet of mesoblast_, split off from the hypoblast
somewhat later than the other two, we should avoid having to admit its
hypoblastic origin.
Professor Huxley, to whom I have shewn my specimens, strongly advocates
this view.
The other possibility is that the notochord is primitively a true
_hypoblastic_ structure which has only by adaptation become an apparently
_mesoblastic_ one in the higher vertebrates. In favour of this view are the
following considerations:
(1) That this is the undoubtedly natural interpretation of the sections.
(2) That the notochord becomes separated from the hypoblast after the
latter has acquired its typical structure, and differs in that respect from
the two lateral sheets of mesoblast, which are formed coincidently with the
hypoblast by a homogeneous mass of cells becoming differentiated into two
distinct layers. (3) That the first mode of looking at the matter really
proves too much, since it is clear that by the same method of reasoning we
could prove the mesoblastic origin of any organ derived from the hypoblast
and budded off into the mesoblast. We would merely have to assert that it
was really a mass of mesoblast budded off from the hypoblast rather later
than the remainder of the mesoblast. Still, it must be admitted that the
first view I have suggested is a possible, not to say a probable one,
though the mode of arguing by which it can be upheld may be rather
dangerous if generally applied. We ought not, however, for that reason
necessarily to reject it in the present case. As Mr Ray Lankester pointed
out to me, if we accept the hypoblastic origin of the notochord, we should
find a partial parallel to it in the endostyle of Tunicates, and it is
perhaps interesting to note in reference to it that the notochord is the
only _unsegmented_ portion of the axial skeleton.
Whether the strong _à priori_ difficulties of the hypoblastic origin of the
notochord are sufficient to counterbalance the natural interpretation of my
sections, cannot, I think, be decided from the single case of the Dog-fish.
It is to be hoped that more complete investigations of the Lamprey, &c.,
may throw further light upon the question.
Whichever view of the primitive origin of the notochord is the true one,
its apparent origin is very instructive as illustrating the possible way in
which an organ might come to change the layer to which it primarily
belonged.
If the notochord is a true mesoblastic structure, it is easy to be seen
how, by becoming separated from the hypoblast a little later than is the
case with the Dog-fish, its mesoblastic origin would become lost; while if,
on the other hand, it is primitively a hypoblastic structure, we see from
higher vertebrates how, by becoming separated from the hypoblast rather
earlier than in the Dog-fish, viz. at the same time as the rest of the
mesoblast, its primitive derivation from the hypoblast has become
concealed.
The view seemingly held by many embryologists of the present day, that an
organ, when it was primitively derived from one layer, can never be
apparently formed in another layer, appears to me both unreasonable on _à
priori_ grounds, and also unsupported by facts.
I see no reason for doubting that the embryo in the earliest periods of
development is as subject to the laws of natural selection as is the animal
at any other period. Indeed, there appear to me grounds for the thinking
that it is more so. The remarkable differences in allied species as to the
amount of food-yolk, which always entail corresponding alterations in the
development--the different modes of segmentation in allied species, such as
are found in the Amphipoda and Isopoda--the suppression of many stages in
freshwater species, which are retained in the allied marine species--are
all instances of modifications due to natural selection affecting the
earliest stages of development. If such points as these can be affected by
natural selection I see no reason why the arrangement of individual cells
(or rather primitive elements) should not also be modified; why, in fact, a
mass of cells which was originally derived from one layer, but in the
course of development became budded off from that layer and entered another
layer, should not by a series of small steps cease ever to be attached to
the original layer, but from the first moment it can be distinguished
should be found as a separate mass in the second layer.
The change of layers will, of course, only take place where some economy is
effected by it. The variations in the mode of development of the nervous
system may probably be explained in this way.
If we admit that organs can undergo changes, as to the primitive layer from
which they are derived, important consequences must follow.
It will, for instance, by no means be sufficient evidence of two organs not
being homologous that they are not developed from the same layer. It
renders the task of tracing out the homologies from development much more
difficult than if the ordinary view of the invariable correspondence of the
three layers throughout the animal kingdom be accepted. Although I do not
believe that this correspondence is invariable or exact, I think that we
both find and should expect to find that it is, roughly speaking, fairly
so.
Thus, the muscles, internal skeleton, and connective tissue are always
placed in the adult between the skin (epidermis) and the epithelium of the
alimentary canal.
We should therefore expect to find them, and, as a matter of fact, we
always do find them, developed from a middle layer when this is present.
The upper layer must always and does always form the epidermis, and
similarly the lower layer or hypoblast must form a part of the epithelium
of the alimentary canal. A full discussion of this question would, however,
lead me too far away from my present subject.
The only other point of interest which I can touch on in this stage is the
commencing closure of the alimentary canal in the region of the head. This
is shewn in Pl. 3, figs. 6_a_, 6_b_, 7_b_, _n.a_. From these figures it
can be seen that the closing does not take place as much by an infolding as
by an ingrowth from the side walls of the alimentary canal towards the
middle line. In this abnormal mode of closing of the alimentary canal we
have again, I believe, an intermediate stage between the mode of formation
of the alimentary canal in the Frog and the typical folding in which occurs
in Birds. There is, however, another point in reference to it which is
still more interesting. The cells to form the ingrowth from the bottom
(ventral) wall of the alimentary canal are derived by a continuous fresh
formation from the yolk, being formed around the nuclei spoken of above
(vide p. 63 et seq.). All my sections shew this with more or less
clearness, especially those a little later than fig. 6_b_, in which the
lower wall of the alimentary canal is nearly completed. This is the more
interesting since, from the mode of formation of the alimentary canal in
the Batrachians, &c., we might expect that the cells from the yolk would
take a share in its formation in the Dog-fish. I have not as yet made out
for certain the share which is taken by these freshly formed cells of the
yolk in the formation of any other organ.
By the completion of its lower wall in the way described, the throat early
becomes a closed tube, its closing taking place before any other important
changes are visible in the embryo from surface views.
A considerable increase in length is attained before other changes than an
increase in depth of the medullary groove and a more complete folding off
of the embryo from the blastoderm take place. The first important change is
the formation of the protovertebræ.
These are formed by the lateral plates of mesoblast, which I said were
equivalent at once to the vertebral and lateral plates in the Bird,
becoming split by transverse divisions into cubical masses.
At the time when this occurs, and, indeed, up till a considerably later
period, the mesoblast is not split into somatopleure and splanchnopleure,
and it is not divided into vertebral and lateral plates. The transverse
lines of division of the protovertebræ do not, however, extend to the outer
edge of the undivided lateral plates.
The differences between this mode of formation of the protovertebræ and
that occurring in Birds are too obvious to require pointing out. I will
speak of them more fully when I have given the whole history of the
protovertebræ of the Dog-fish.
I will only now say that I have had in the early stages to investigate the
formation of the protovertebræ entirely by means of sections, the objects
being too opaque to be otherwise studied.
The next change of any importance is the commencement of the formation of
the head. The region of the head first becomes distinguishable by the
flattening out of the germ at its front end.
The flattened-out portion of the germ grows rapidly, and forms a
spatula-like termination to the embryo (Pl. 3, fig. 8).
In the region of the head the medullary groove is at first totally absent
(vide section, Pl. 3, fig. 8_a_).
Indeed, as can be seen from fig. 8_b_, the laminæ dorsales, so far from
bending up at this stage, actually bend down in the opposite direction.
I am at present quite unable even to form a guess what this peculiar
feature of the brain means. It, no doubt, has some meaning in reference to
the vertebrate ancestry if we could only discover it. The peculiar
spatula-like flattened condition of the head is also (vide _loc. ant.
cit._) apparently found in the Sturgeons; it must therefore almost
undoubtedly be looked upon as not merely an accidental peculiarity.
While these changes have been taking place in the head not less important
changes have occurred in the remainder of the body. In the first place the
two caudal lobes have increased in size, and have become, as it were,
pushed in together, leaving a groove between them (fig. 8, _ts_). They are
very conspicuous objects, and, together with the spatula-like head, give
the whole embryo an almost comical appearance. The medullary canal has by
this time become completely closed in the region of the tail (figs. 8 and
8_b_).
It is still widely open in the region of the back, and, though more nearly
closed again in the neck, is, as I have said, flattened out to nothing in
the head.
The groove[15] between the two caudal lobes must not be confused (as may
easily be done) with the medullary groove, which by the time the former
groove has become conspicuous is a completely closed canal.
Footnote 15: This groove is the only structure which it seems
possible to compare with the so-called "primitive groove" of
Birds. It is, however, doubtful whether they are really
homologous.
The vertebral plates are not divided (vide fig. 7) into a somatopleuric and
splanchnopleuric layer by this stage, except in the region of the head
(vide fig. 8_b_, _pp_), where there is a distinct space between the two
layers, which is undoubtedly homologous with the pleuro-peritoneal cavity
of the hinder portion of the body.
It is probably the same cavity which Oellacher (_loc. cit._) calls in
Osseous fishes the pericardial cavity. In the Dog-fish, at least, it has no
connection with the pericardium. Of its subsequent history I shall say a
few words when I come to speak of the later stages.
The embryo does not take more than twenty-four hours in passing from this
stage, when the head is a flat plate, to the stage when the whole neural
canal (including the region of the head) is closed in. The other changes,
in addition to the closing in of the neural canal, are therefore somewhat
insignificant. The folding off of the embryo from the germ has, however,
progressed considerably, and a portion of the hind gut is closed in below.
This is accomplished, not by a tail-fold, as in Birds, but by two lateral
folds, which cause the sides of the body to meet and coalesce below. At the
extreme hind end, where the epiblast is continuous with the hypoblast, the
lateral folds turn round, so to speak, and become continuous with the
medullary folds, so that when the various folds meet each other an
uninterrupted canal is found passing round from the neural into the
alimentary canal, and placing these two in communication at the tail end of
the body. Since I have already mentioned this, and spoken of its
significance, I will not dwell on it further here.
The cranial flexure commences coincidently with the closing in of the
neural canal in the region of the brain, and the division into fore, mid,
and hind brain becomes visible at the same time as or even before the
closing of the canal occurs. The embryo has now become more or less
transparent, and protovertebræ, of which about twenty are present, can
_now_ be seen in the fresh specimens. The heart, however, is not yet
formed.
Up to this period, a period at which the embryo becomes very similar in
external appearance to any other vertebrate embryo, I have followed in my
description a chronological order. I shall now cease to do so, since it
would be too long for a preliminary notice of this kind, but shall confine
myself to the history of a few organs whose development is either more
important or more peculiar than that of the others.
_The Protovertebræ._
I have thought it worth while to give a short history of the development of
the protovertebræ, firstly, because it is very easy to follow this in the
Dog-fish, and, secondly, because I believe that the Dog-fish have more
nearly retained the primitive condition of the protovertebræ than any other
vertebrate whose embryology has hitherto been described with sufficient
detail.
I intend to describe, at the same time, the development of the spinal
nerves.
I left each lateral mass of mesoblast in my last stage as a plate which had
not yet become split into a somatic and a splanchnic sheet (Pl. 3,
fig. 8_a_, _vp_), but which had become cut by transverse lines (not,
indeed, extending to the outer limit of the sheet, but as yet not cut off
by longitudinal lines of cleavage) into segments, which I called
protovertebræ.
This sheet of mesoblast is fairly thick at its proximal (upper) end, but
thins off laterally to a sheet two cells deep, and its cells are so
arranged as to foreshadow its subsequent splitting into somatic and
splanchnic sheets. Its upper (proximal) end is at this stage level with the
bottom of the neural canal, but soon begins to grow upwards, and at the
same time the splitting into somatopleure and splanchnopleure commences
(Pl. 3, fig. 10, _so_ and _sp_).
The separation between the two sheets is first visible in its uppermost
part, and thence extends outwards. By this means each of the protovertebræ
becomes divided into two sheets, which are only connected at their upper
ends and outside the region of the body. I speak of the whole lateral sheet
as being composed of protovertebræ, because at this time no separation into
vertebral and lateral plates can be seen; but I may anticipate matters by
saying that only the upper portion of the sheet from the level of the top
of the digestive canal, becomes subsequently the true protovertebræ. From
this it is clear that the pleuro-peritoneal cavity extends primitively
quite up to the top of the protovertebræ; and that thus a portion of a
sheet of mesoblast, at first perfectly continuous with the splanchnic sheet
from which is derived the muscular wall of the alimentary canal, is
converted into a part of the voluntary muscular system of the body, having
no connection whatever with the involuntary muscular system of the
digestive tract.
The pleuro-peritoneal cavity is first distinctly formed at a time when only
two visceral clefts are present. Before the appearance of a third visceral
cleft in a part of the innermost layer of each protovertebræ (which may be
called the splanchnic layer, from its being continuous with the mesoblast
of the splanchnopleure), opposite the bottom of the neural tube, some of
the cells commence to become distinguishable from the rest, and to form a
separate mass. This mass becomes much more distinct a little later, its
cells being characterised by being spindle-shaped, and having an elongated
nucleus which becomes deeply stained by reagents (Pl. 4, fig. 11, _mp´_).
Coincidently with its appearance the young Dog-fish commences spontaneously
to move rapidly from side to side with a kind of serpentine motion, so
that, even if I had not traced the development of this differentiated mass
of cells till it becomes a band of muscles close to the notochord, I should
have had little doubt of its muscular nature. It is indicated in figs. 11,
12, 13, by the letters _mp´_. Its early appearance is most probably to be
looked upon as an adaptation consequent upon the respiratory requirements
of the young Dog-fish necessitating movements within the egg.
Shortly after this date, at a period when three visceral clefts are
present, I have detected the first traces of the spinal nerves.
At this time they appear in sections as small elliptical masses of cells,
entirely independent of the protovertebræ, and closely applied to the upper
and outer corners of the involuted epiblast of the neural canal (Pl. 4,
fig. 11, _spn_). These bodies are far removed from any mesoblastic
structures, and at the same time the cells composing them are _not_ similar
to the cells composing the walls of the neural canal, and are not attached
to these, though lying in contact with them. I have not, therefore,
sufficient evidence at present to enable me to say with any certainty where
the spinal nerves are derived from in the Dog-fish. They may be derived
from the involuted epiblast of the neural canal, and, indeed, this is the
most natural interpretation of their position.
On the other hand, it is possible that they are formed from wandering cells
of the mesoblast--a possibility which, with our present knowledge of
wandering cells, must not be thrown aside as altogether improbable.
In any case, it is clear that the condition in the Bird, where the spinal
nerves are derived from tissue of the protovertebræ, is not the primitive
one. Of this, however, I will speak again when I have concluded my account
of the development of the protovertebræ.
About the same time that the first rudiments of the nerves appear, the
division of the mesoblast of the sides of the body into a vertebral and a
lateral portion occurs. This division first appears in the region where the
oviduct (Müller's duct) is formed (Pl. 4, fig. 11, _ov_).
At this part opposite the level of the dorsal aorta the two sheets, viz.
the splanchnic and the somatic, unite together, and thus each lateral sheet
of mesoblast becomes divided into an upper portion (fig. 11, _mp_), split
up by transverse partitions into protovertebræ, and a lower portion not so
split, but consisting of an outer layer, the true somatopleure, and an
inner layer, the true splanchnopleure. These two divisions of the primitive
plate are thus separated by the line at which a fusion between the
mesoblast of the somatopleure and splanchnopleure takes place. The mass of
cells resulting from the fusion at this point corresponds with the
intermediate cell-mass of Birds (vide Waldeyer, _Eierstock und Ei_).
At the same time, in the upper of these two sheets (the protovertebræ), the
splanchnic layer sends a growth of cells inwards towards the notochord and
the neural canal. This growth is the commencement of the large quantity of
mesoblastic tissue around the notochord, which is in part converted into
the axial skeleton, and in part into the connective tissue adjoining this.
This mass of cells is at first quite continuous with the splanchnic layer
of the protovertebræ, and I see no reason for supposing that it is not
derived from the growth of the cells of this layer. The ingrowth to form it
first appears a little after the formation of the dorsal aorta; but, as far
as I have been able to see, its cells have no connection with the walls of
the aorta.
What I have said as to the development of the skeleton-forming layer will
be quite clear from figs. 11 and 12_a_; and from these it will also be
clear, especially from fig. 11_a_, that the outermost layer of this mass of
cells, which was the primitive splanchnic layer of the protovertebræ, still
retains its epithelial character, and so can easily be distinguished from
those cells which will form the skeleton. In the next stage which I have
figured (fig. 12_a_), this outer portion of the splanchnic layer is
completely separated from the skeleton-forming cells, and at the same time,
having united below as well as above with the outer (somatic) layer of the
two layers of which the protovertebræ are formed, the two together form an
independent mass (fig. 12, _mp_), similar in appearance and in every way
homologous with the muscle-plate of Birds.
On the inner side of this, which we may now call the muscle-plate, is seen
the bundle of earlier-developed muscles (fig. 12, _mp´_) which I spoke of
before.
The section represented in fig. 12 is from a very considerably later embryo
than that represented in fig. 11, so that the skeleton-forming cells, few
in number in the earlier section, have become very numerous in the later
one, and have grown up above the neural canal, and also below the
notochord, between the digestive canal and the aorta. They have, moreover,
changed their character; they were round before, now they have become
stellate. As to their further history, it need only be said that the layer
of them immediately around the notochord and neural canal forms the
cartilaginous centra and arches of the vertebræ, and that the remaining
portion of them, which becomes much more insignificant in size as compared
with the muscles, forms the connective tissue of the skeleton and of the
parts around and between the muscles.
A muscle-plate itself is at this stage (shewn in fig. 12) composed of an
inner and an outer layer of columnar cells (splanchnic and somatic) united
at the upper and lower ends of the plate, and on the inner of the two lies
the more developed mass of muscles before spoken of (_mp´_).
Each of these plates now grows both upwards and downwards; and at the same
time connective-tissue cells appear between the plates and epidermis; but
from where they come I do not know for certain; very probably they are
derived from the somatic layer of the muscle-plate.
While the muscle-plates continue to grow both upwards and downwards, the
cells of which they are composed commence to become elongated and soon
acquire an unmistakably muscular character (Pl. 4, fig. 13, _mp_).
Before this has occurred the inner mass of muscles has also undergone
further development and become a large and conspicuous band of muscles
close to the notochord (fig. 13, _mp´_).
At the same time that the muscle-plates acquire the true histological
character of muscle, septa of connective tissue grow in and divide them
into a number of distinct segments which subsequently form separate bands
of muscle. I will not say more in reference to the development of the
muscular system than that the whole of the muscles of the body (apart from
the limbs, the origin of whose muscular system I have not yet investigated)
are derived from the muscle-plates which grow upwards above the neural
canal and downwards to the ventral surface of the body.
During the time the muscle-plates have been undergoing these changes the
nerve masses have also undergone developmental changes.
They become more elongated and fibrous, their main attachment to the neural
tube being still at its posterior (dorsal) surface, near which they first
appeared. Later still they become applied closely to the sides of the
neural tube and send fibres to it below as well as above. Below (ventral
to) the neural tube a ganglion appears, forming only a slight swelling, but
containing a number of characteristic nerve-cells. The ganglion is
apparently formed just below the junction of the anterior and posterior
roots, though probably the fibres of the two roots do not mix till below
it.
The main points which deserve notice in the development of the
protovertebræ are--
(1) That at the time when the mesoblast becomes split horizontally into
somatopleure and splanchnopleure the vertebral and lateral plates are one,
and the splitting extends to the very top of the vertebral or muscle-plate,
so that the future muscle-plates are divided into a splanchnic and somatic
layer, the space between which is at first continuous with the
pleuro-peritoneal cavity.
(2) That the following parts are respectively formed by the vertebral and
lateral plates:
(_a_) Vertebral plate. From the splanchnic layer of this, or from cells
which appear close to and continuous with it, the skeleton, and connective
tissue of the upper part of the body, are derived.
The remainder of the plate, consisting of a splanchnic and somatic layer,
is entirely converted into the muscles of the trunk, all of which are
derived from it.
(_b_) Between the vertebral plate and the lateral plate is a mass of cells
where, as I mentioned above, the mesoblast of the somatopleure and
splanchnopleure fuse together. This mass of cells is the equivalent of the
_intermediate cell_ mass of Birds (vide Waldeyer, _Eierstock und Ei_).
From it are derived the Wolffian bodies and duct, the oviduct, the ovaries
and the testis, and the connective tissue of the parts adjoining these.
(_c_) The lateral plate. From the somatic layer of this is derived the
connective tissue of the ventral half of the body; the mesoblast of the
limbs, including probably the muscles, and certainly the skeleton. From its
splanchnic layer are derived the muscles and connective tissue of the
alimentary canal.
(3) The spinal nerves are developed independently of the protovertebræ, so
that the protovertebræ of the Elasmobranchii do not appear to be of such a
complicated structure as the protovertebræ of Birds.
_The Digestive Canal._
I do not intend to enter into the whole history of the digestive canal, but
to confine myself to one or two points of interest connected with it. These
fall under two heads:
(1) The history of the portion of the digestive canal between the anus and
the end of the tail where the digestive canal opens into the neural canal.
(2) Certain less well-known organs derived from the digestive canal.
The anus is a rather late formation, but its position becomes very early
marked out by the hypoblast of the digestive canal approaching at that
point close to the surface, whilst receding to some little distance from it
on either side. The portion of the digestive tract I propose at present
dealing with is that between this point, which I will call, for the sake of
brevity, the anus and the hind end of the body. This portion of the canal
is at first very short; it is elliptical in section, and of rather a larger
bore than the remainder of the canal. Its diameter becomes, however,
slightly less as it approaches the tail, dilating again somewhat at its
extreme end. It is lined by a markedly columnar epithelium. Though at first
very short, its length increases with the growth of the tail, but at the
same time its calibre continually becomes smaller as compared with the
remainder of the alimentary canal.
It commences to become smaller, first of all, near, though not quite, at
its extreme hind end, and thus becomes of a conical shape; the base of the
cone being just behind the anus, while the apex of the cone is situated a
short distance from the hind end of the embryo. The extreme hind end,
however, at the same time does not diminish in size, and becomes relatively
(if not also absolutely) much larger in diameter than it was at first, as
compared with the remainder of the digestive canal. It becomes, in fact, a
vesicle or vesicular dilatation at the end of a conical canal.
Just before the appearance of the external gills this part of the digestive
canal commences to atrophy. It begins to do so close to the terminal
vesicle, which, however, still remains as or more conspicuous than it was
before. The lumen of the canal becomes smaller and smaller, and finally it
becomes a solid string of cells, and these also soon become
indistinguishable and not a trace of the canal is left.
Almost the whole of it has disappeared before the vesicle begins to
atrophy, but very shortly after all trace of the rest of the canal has
vanished the terminal vesicle also vanishes. This occurs just about the
time or shortly after the appearance of the external gills--there being
slight differences probably in this respect in the different species.
In this history there are two points of especial interest:
(1) The terminal vesicle.
(2) The disappearance of a large and well-developed portion of the
alimentary canal.
The interest in the terminal vesicle lies in the possibility of its being
some rudimentary structure.
In Osseous fishes Kupffer has described the very early appearance of a
vesicle near the tail end, which he doubtfully speaks of as the
"allantois." The figure he gives of it in his earlier paper (_Archiv. für
Micro. Anat._ Vol. II. pl. xxiv, fig. 2) bears a very strong resemblance to
my figures of this vesicle at the time when the hind end of the alimentary
canal is commencing to disappear; and I feel fairly confident that it is
the same structure as I have found in the Dog-fish: but until the relations
of the Kupffer's vesicle to the alimentary canal are known, any comparison
between it and the terminal vesicle in the Dog-fish must be to a certain
extent guess-work.
I have, however, been quite unsuccessful in finding any other vesicular
structure which can possibly correspond to the so-called allantoic vesicle
of Osseous fish.
The disappearance of a large portion of the alimentary canal behind the
anus is very peculiar. In order, however, to understand the whole
difficulties of the case I shall be obliged to speak of the relations of
the anus of the Dog-fish to the anus of Rusconi in the Lamprey, &c.
In those vertebrates whose alimentary canal is formed by an involution, the
anus of Rusconi represents the opening of this involution, and therefore
the point where the alimentary canal primitively communicates with the
exterior. When, however, the "anus of Rusconi" becomes _closed_, the wall
of the alimentary canal still remains at that point in close juxtaposition
to the surface, and the new and final anus is formed at or close to that
point. In the Dog-fish, although the anus of Rusconi is not present, still,
during the closing of the alimentary canal, the point which would
correspond with this becomes marked out by the alimentary canal there
approaching the surface, and it is at this point that the involution to
form the true anus subsequently appears.
The anus in the Dog-fish has thus, more than a mere secondary significance.
It corresponds with the point of closing of the primitive involution. If it
was not for this peculiarity of the vertebrate anus we would naturally
suppose, from the disappearance of a considerable portion of the alimentary
canal lying behind its present termination, that in the adult the
alimentary canal once extended much farther back than at present, and that
the anus we now find was only a secondary anus, and not the primitive one.
It is perhaps possible that this hinder portion of the alimentary canal is
a result of the combined growth of the tail and the persisting continuity
(at the end of the body) of the epiblast with the hypoblast.
Whichever view is correct, it may be well to mention, in order to shew that
the difficulty about the anus of Rusconi is no mere visionary one, that
Götte ("Untersuchung über die Entwicklung der Bombinator igneus," _Archiv.
für Micro. Anat._, vol. V. 1869) has also described the disappearance of
the hind portion of the alimentary canal in Batrachians, a rudiment
(according to him) remaining in the shape of a lymphatic trunk.
It is, perhaps, possible that we have a further remnant of this "hind
portion" of the alimentary canal amongst the higher vertebrates in the
"allantois."
_Organs developed from the Digestive Canal._
In reference to the development of the liver, pancreas, &c., as far as my
observations have at present gone, the Dog-fish presents no features of
peculiar interest. The liver is developed as in the Bird, and independently
of the yolk.
There are, however, two organs derived from the hypoblast which deserve
more attention. Immediately under the notochord, and in contact with it
(vide Pl. 3, fig. 10; Pl. 4, figs. 11 and 12, _x_), a small roundish (in
section) mass of cells is to be seen in most of the sections.
Its mode of development is shewn in fig. 10, _x_. That section shows a mass
of cells becoming pinched off from the top of the alimentary canal. By this
process of pinching off from the alimentary canal a small rod-like body
close under the notochord is formed. It persists till after the appearance
of the external gills, but later than that I have not hitherto succeeded in
finding any trace of it.
It was first seen by Götte (_loc. cit._) in the Batrachians, and he gave a
correct account of its development, and added that it became the thoracic
duct.
I have not myself worked out the later stages in the development of this
body with sufficient care to be in a position to judge of the correctness
of Götte's statements as to its final fate. If it is true that it becomes
the thoracic duct it is very remarkable, and ought to throw some light upon
the homologies of the lymphatic system.
Some time before the appearance of the external gills another mass of cells
becomes, I believe, constricted off from the part of the alimentary canal
in the neighbourhood of the anus, and forms a solid rod composed at first
of dark granular cells lying between the Wolffian ducts. I have not
followed out its development quite completely, but I have very little doubt
that it is really constricted off from a portion of the alimentary canal
chiefly in front of the point where the anus appears, but also, I believe,
from a small portion behind this.
Though the cells of which it is composed are at first columnar and granular
(fig. 12, _su_, _r_), they soon begin to become altered, and in the latter
stage of its development the body forms a conspicuous rounded mass of cells
with clear protoplasm, and each provided with a large nucleus. Later still
it becomes divided into a number of separate areas of cells by septa of
connective tissue, in which (the septa) capillaries are also present. Since
I have not followed it to its condition in the adult, I cannot make any
definite statements as to the fate of this body; but I think that it
possibly becomes the so-called suprarenal organ, which in the Dog-fish
forms a yellowish elongated body lying between the two kidneys.
_The development of the Wolffian Duct and Body and of the Oviduct._
The development of the Wolffian duct and the Oviduct in the various classes
of vertebrates is at present involved in some obscurity, owing to the very
different accounts given by different observers.
The manner of development of these parts in the Dog-fish is different from
anything that previous investigators have met with in other classes, but I
believe that it gives a clearer insight into the true constitution of these
parts than vertebrate embryology has hitherto supplied, and at the same
time renders easier the task of understanding the differences in the modes
of development in the different classes.
I shall commence with a simple description of the observed facts, and then
give my view as to their meaning. At about the time of the appearance of
the third visceral cleft, and a short way behind the point up to which the
alimentary canal is closed in front, the splanchnopleure and somatopleure
fuse together opposite the level of the dorsal aorta.
From the mass of cells formed by this fusion a solid knob rises up towards
the epiblast (Pl. 4, fig. 11_b_, _ov_), and from this knob a solid rod of
cells grows backwards towards the tail (fig. 11_c_, _ov_) very closely
applied to the epiblast. This description will be rendered clear by
referring to figs. 11_b_ and _c_. Fig. 11_b_ is a section at the level of
the knob, and fig. 11_c_ is a section of the same embryo a short way behind
this point. So closely does the rod of cells apply itself to the epiblast
that it might very easily be supposed to be derived from it. Such, indeed,
was at first my view till I cut a section passing through the knob. In
order, however, to avoid all possibility of mistake I made sections of a
large number of embryos of about the age at which this appears, and
_invariably found_ the large knob in front, and from it the solid string
growing backwards.
This string is the commencement of the _Oviduct_ or _Müller's duct_, which
in the Dog-fish as in the Batrachians is the first portion of the
genito-urinary system to appear, and is in the Dog-fish undoubtedly at
first solid. All my specimens have been hardened with osmic acid, and with
specimens hardened with this reagent it is quite easy to detect even the
very smallest hole in a mass of cells.
As a solid string or rod of cells the Oviduct remains for some time; it
grows, indeed, rapidly in length, the extreme hind end of the rod being
very small and the front end continuing to remain attached to the knob. The
knob, however, travels inwards and approaches nearer and nearer to the true
pleuro-peritoneal cavity, always remaining attached to the intermediate
cell mass.
At about the time when five visceral clefts are present the Oviduct first
begins to get a lumen and to open at its front end into the
pleuro-peritoneal cavity. The cells of the rod are first of all arranged in
an irregular manner, but gradually become columnar and acquire a radiating
arrangement around a central point. At this point, where the ends of all
the cells meet, a very small hole appears, which gradually grows larger and
becomes the cavity of the duct (fig. 12, _ov_). The hole first makes its
appearance at the anterior end of the duct, and then gradually extends
backwards, so that the hind end is still without a lumen, when the lumen of
the front end is of a considerable size.
At the front knob the same alteration in the cells takes place as in the
rest of the duct, but the cells become deficient on the side adjoining the
pleuro-peritoneal cavity, so that an opening is formed into the
pleuro-peritoneal cavity, which soon becomes of a considerable size. Soon
after its first formation, indeed, the opening becomes so large that it may
be met in from two to three consecutive sections if these are very thin.
Thus is formed the lumen of the Oviduct. The duct still, at this age, ends
behind without having become attached to the cloaca, so that at this time
the Oviduct is a canal closed behind, but communicating in front by a large
opening with the pleuro-peritoneal cavity.
It has during this time been travelling downwards, and is now much nearer
the pleuro-peritoneal cavity than the epiblast.
It may be well to point out that the mode of development which I have
described is really not very different from an involution, and must, in
fact, be only looked upon as a modification of an involution. Many examples
from all classes in the animal kingdom could be selected to exemplify how
an involution may become simply a solid thickening. In the Osseous fish
nearly all the organs which are usually formed by an involution have
undergone this change in their mode of development. I shall attempt to give
reasons later on for the solid form having been acquired in this particular
case of the Oviduct.
At about the time when a lumen appears in the Oviduct the first traces of
the Wolffian duct become visible.
At intervals along the whole length, between the front and hind ends of the
Oviduct, involutions arise from the pleuro-peritoneal cavity (fig. 12_a_,
_pwd_) on the inside (nearer the middle line) of the Oviduct. The upper
ends of these numerous involutions unite together and form a string of
cells, at first solid, but very soon acquiring a lumen, and becoming a duct
which communicates (as it clearly must from its mode of formation), at
numerous points with the pleuro-peritoneal cavity. It is very probable that
there is one involution to each segment of the body between the front and
hind ends of the Oviduct. This duct is the Wolffian duct, which thus,
together with the Oviduct, is formed before the appearance of the external
gills.
For a considerable period the front end of the Oviduct does not undergo
important changes; the hind end, however, comes into connection with the
extreme end of the alimentary canal. The two Oviducts do not open together
into the cloaca, though, as my sections prove, their openings are very
close together. The whole Oviduct, as might be expected, shares in the
general growth, and its lumen becomes in both sexes very considerably
greater than it was before.
It is difficult to define the period at which I find these changes
accomplished without giving drawings of the whole embryo. The stage is one
considerably after the external gills have appeared, but before the period
at which the growth of the olfactory bulbs renders the head of an elongated
shape.
During the same period the Wolffian duct has undergone most important
changes. It has commenced to bud off diverticula, which subsequently become
the tubules of the Wolffian body (vide fig. 13, _wd_). I am fairly
satisfied that the tubules are really budded off, and are not formed
independently in the mesoblast. The Dog-fish agrees so far with Birds,
where I have also no doubt the tubules of the Wolffian body are formed as
diverticula from the Wolffian duct.
The Wolffian ducts have also become much longer than the Oviduct, and are
now found behind the anus, though they do not extend as far forward as does
the Oviduct.
They have further acquired a communication with the Oviduct, in the form of
a narrow duct passing from each of them into an Oviduct a short way before
the latter opens into the cloacal dilatation of the alimentary canal.
The canals formed by the primitive involution leading from the
pleuro-peritoneal cavity into the Wolffian duct have become much more
elongated, and at the same time narrower. One of these is shewn in fig. 13,
_pwd_.
Any doubt which could possibly be entertained as to the true character of
the ducts whose development I have described is entirely removed by the
development of the tubules of the Wolffian body. In the still later stage
than this further proofs are furnished involving the function of the
Oviduct. At the period when the olfactory lobes have become so developed as
to render the head of the typical elongated shape of the adult, I find that
the males and females can be distinguished by the presence in the former of
the clasping appendages[16]. I find at this stage that in the female the
front ends of the Oviducts have approached the middle line, dilated
considerably, and commenced to exhibit at their front ends the
peculiarities of the adult. In the male they are much less conspicuous,
though still present.
Footnote 16: For the specimens of this age I am indebted to
Professor Huxley.
At the same time the tubules of the Wolffian body become much more
numerous, the Malpighian tufts appear, and the ducts cease almost, if not
entirely, to communicate with the pleuro-peritoneal cavity. I have not made
out anything very definitely as to the development of the Malpighian tufts,
but I am inclined to believe that they arise independently in the mesoblast
of the intermediate cell mass.
The facts which I have made out in reference to the development of the
Wolffian duct, especially of its arising as a _series of involutions_ from
the pleuro-peritoneal cavity, will be found, I believe, of the greatest
importance in understanding the true constitution of the Wolffian body. To
this I will return directly, but first wish to clear the ground by
insisting upon one preliminary point.
From their development the Oviduct and Wolffian body appear to stand to
each other in the relation of the Wolffian duct being the equivalent to a
series, so to speak, of Oviducts.
I pointed out before that the mode of development of the Oviduct could only
be considered as a modification of a simple involution from the
pleuro-peritoneal cavity. Its development, both in the Birds and in the
Batrachians as an involution, still more conclusively proves the truth of
this view.
The explanation of its first appearing as a solid rod of cells which keeps
close to the epiblast is, I am inclined to think, the following. Since the
Oviduct had to grow a long way backwards from its primitive point of
involution, it was clearly advantageous for it not to bore its way through
the mesoblast of the intermediate cell mass, but to pass between this and
the epiblast. This modification having been adopted, was followed by the
knob forming the origin of the duct coming to be placed at the outside of
the intermediate cell mass rather than close to the pleuro-peritoneal
cavity, a change which necessitated the mode of development by an
involution being dropped and the solid mode of development substituted for
it, a lumen being only subsequently acquired.
In support of the modification in the development being due to this cause
is the fact that in Birds a similar modification has taken place with the
Wolffian duct. The Wolffian duct there arises differently from its mode of
development in all the lower vertebrates as a solid rod close to the
epiblast[17], instead of as an involution.
Footnote 17: If Romiti's observations (_Archives für Mikr.
Anatom._ Vol. IX. p. 200) are correct, then the ordinary view
of the Wolffian duct arising in Birds as a solid rod at the
outer corner of the protovertebræ will have to be abandoned.
If the above explanation about the Oviduct be correct, then it is clear
that similar causes have produced a similar modification in development
(only with a different organ) in Birds; while, at the same time, the
primitive mode of origin of the Oviduct (Müller's duct) has been retained
by them.
The Oviduct, then, may be considered as arising by an involution from the
pleuro-peritoneal cavity.
The Wolffian duct arises by a series of such involutions, all of which are
behind (nearer the tail) the involution to form the Oviduct.
The natural interpretation of these facts is that in the place of the
Oviduct and Wolffian body there were primitively a series of similar bodies
(probably corresponding in number with the vertebral segments), each
arising by an involution from the pleuro-peritoneal cavity; and that the
first of these subsequently became modified to carry eggs, while the rest
coalesced to form the Wolffian duct.
If we admit that the Wolffian duct is formed by the coalescence of a series
of similar organs, we shall only have to extend the suggestion of Gegenbaur
as to the homology of the Wolffian body in order to see its true nature.
Gegenbaur looks upon the whole urinogenital system as homologous with a
pair of segmental organs. Accepting its homology with the segmental organs,
its development in Elasmobranchii proves that it is not one pair, but a
series of pairs of segmental organs with which the urinogenital system is
homologous. The first of these have become modified so as to form the
Oviducts, and the remainder have coalesced to form the Wolffian ducts.
The part of a segmental organ which opens to the exterior appears to be
lost in the case of all but the last one, where this part is still
retained, and serves as the external opening for all.
Whether the external opening of the first segmental organ (Oviduct) is
retained or not is doubtful. Supposing it has been lost, we must look upon
the external opening for the Wolffian body as serving also for the Oviduct.
In the case of all other vertebrates whose development has been
investigated (but the Elasmobranchii), the Wolffian duct arises by a single
involution, or, what is equivalent to it, the other involutions having
disappeared. This even appears to be the case in the Marsipobranchii. In
the adult Lamprey the Wolffian duct terminates at its anterior end by a
large ciliated opening into the pleuro-peritoneal cavity. It will, perhaps,
be found, when the development of the Marsipobranchii is more carefully
studied, that there are _primitively_ a number of such openings[18]. The
Oviduct, when present, arises in other vertebrates as a single involution,
strongly supporting the view that its mode of formation in the Dog-fish is
fundamentally merely an involution.
Footnote 18: While correcting the proofs of this paper I have
come across a memoir of W. Müller ("Ueber die Persistenz der
Urniere bei Myxine Glutinosa," _Jenaische Zeitschrift_,
Vol. VII. 1873), in which he mentions that in Myxine the upper
end of the Wolffian duct communicates by numerous openings with
the pleuro-peritoneal cavity; this gives to the suggestion in
the text a foundation of fact.
The duct of the testes is, I have little doubt, derived from the anterior
part of the Wolffian body; if so, it must be looked upon as not precisely
equivalent to the Oviduct, but rather to a series of coalesced organs, each
equivalent to the Oviduct. The Oviduct is in the Elasmobranchii, as in
other vertebrates, primitively developed in both sexes. In the male,
however, it atrophies. I found it still visible in the male Torpedos,
though much smaller than in the females near the close of intra-uterine
life.
Whether or not these theoretical considerations as to the nature of the
Wolffian body and Oviduct are correct, I believe that the facts I have
brought to light in reference to the development of these parts in the
Dog-fish will be found of service to every one who is anxious to discover
the true relations of these parts.
Before leaving the subject I will say one or two words about the
development of the Ovary. In both sexes the germinal epithelium (fig. 13)
becomes thickened below the Oviduct, and in both sexes a knob (in section
but really a ridge) comes to project into the pleuro-peritoneal cavity on
each side of the mesentery (fig. 13, _pov_). In both sexes, but especially
the females, the epithelium on the upper surface of this ridge becomes very
much thickened, whilst subsequently it elsewhere atrophies. In the females,
however, the thickened epithelium on the knob grows more and more
conspicuous, and develops a number of especially large cells with large
nuclei, precisely similar to Waldeyer's (_loc. cit._) "primitive ova" of
the Bird. In the male the epithelium on the ridge, though containing
primitive ova, is not as conspicuous as in the female. Though I have not
worked out the matter further than this at present, I still have no doubt
that these projecting ridges become the Ovaries.
_The Head._
The study of the development of the parts of the head, on account of the
crowding of organs which occurs there, always presents greater difficulties
to the investigator than that of the remainder of the body. My observations
upon it are correspondingly incomplete. I have, however, made out a few
points connected with it in reference to some less well-known organs, which
I have thought it worth while calling attention to in this preliminary
account.
_The continuation of the Pleuro-peritoneal Cavity into the Head._
In the earlier part of this paper (p. 86) I called attention to the
extension of the separation between somatopleure and splanchnopleure into
the head, forming a space continuous with the pleuro-peritoneal cavity
(Pl. 3, fig. 8_a_, _pp_); this becomes more marked in the next stage, and,
indeed, the pleuro-peritoneal cavity is present for a considerable time in
the head before it becomes visible elsewhere. At the time of the appearance
of the second visceral cleft it has become for the most part atrophied, but
there persist two separated portions of it in front of the first cleft, and
also remnants of it less well marked between and behind the two clefts. The
visceral clefts necessarily divide it into separate parts.
The two portions in front of the first visceral cleft remain very
conspicuous till the appearance of the external gills, and above the hinder
one of the two the fifth nerve bifurcates.
These two are shewn as they appear in a surface view in fig. 14, _pp_. They
are in reality somewhat flattened spaces, lined by a mesoblastic
epithelium; the epithelium on the inner surface of the space corresponding
to the splanchnopleure, and that on the outer to the somatopleure.
I have not followed the history of these later than the time of the
appearance of the external gills.
The presence of the pleuro-peritoneal cavity in the head is interesting, as
shewing the fundamental similarity between the head and the remainder of
the body.
_The Pituitary Body._
All my sections seem to prove that it is a portion of the epiblastic
involution to form the mouth which is pinched off to form the pituitary
body, and not a portion of the hypoblast of the throat. Since Götte
(_Archiv. für Micr. Anat._ Bd. IX.) has also found that the same is the
case with the Batrachians and Mammalia, I have little doubt it will be
found to be universally the case amongst vertebrates.
Probably the observations which lead to the supposition that it was the
throat which was pinched off to form the pituitary body were made after the
opening between the mouth and throat was completed, when it would naturally
be impossible to tell whether the pinching off was from the epiblast of the
mouth involution or the hypoblast of the throat.
_The Cranial Nerves._
The cranial nerves in their early condition are so clearly visible that I
have thought it worth while giving a figure of them, and calling attention
to some points about their embryonic peculiarities.
From my figure (14) it will be seen that there is behind the auditory
vesicle a nervous tract, from which four nerves descend, and that each of
these nerves is distributed to the front portion of a visceral arch. When
the next and last arch (in this species) is developed, a branch from this
nervous mass will also pass down to it. That each of these is of an equal
morphological value can hardly be doubted.
The nerve to the third arch becomes the glosso-pharyngeal (fig. 14, _gl_),
the nerves to the other arches become the branchial branches of the vagus
nerve (fig. 14, _vg_). Thus the study of their development strongly
supports Gegenbaur's view of the nature of the vagus and glosso-pharyngeal,
viz. that the vagus is a compound nerve, each component part of it which
goes to an arch being equivalent to one nerve, such as the
glosso-pharyngeal.
Of the nerves in front of the auditory sac the posterior is the seventh
nerve (fig. 14, VII). Its mode of distribution to the second arch leaves
hardly a doubt that it is equivalent to one such nerve as those distributed
to the posterior arches. Subsequently it acquires another branch, passing
forwards towards the arch in front.
The most anterior nerve is the fifth (fig. 14, V), of which two branches
are at this stage developed. The natural interpretation of its present
condition is, that it is equivalent to two nerves, but the absence of
relation in its branches to any visceral clefts renders it more difficult
to determine the morphology of the fifth nerve than of the other nerves.
The front branch of the two is the ophthalmic branch of the adult, and the
hind branch the inferior maxillary branch. The latter branch subsequently
gives off low down, _i.e._ near its distal extremity, another branch, the
superior maxillary branch.
In its embryonic condition this latter branch does not appear like a third
branch of the fifth, equivalent to the seventh or the glosso-pharyngeal
nerves, but rather resembles the branch of the seventh nerve which passes
to the arch in front, which also is present in all the other cranial
nerves.
_Modes of Preparation._
Before concluding I will say one or two words as to my modes of
preparation.
I have used picric and chromic acids, both applied in the usual way; but
for the early stages I have found osmic acid by far the most useful
reagent. I placed the object to be hardened, in osmic acid (half per cent.)
for two hours and a half, and then for twenty four in absolute alcohol.
I then embedded and cut sections of it in the usual way, without staining
further.
I found it advantageous to cut sections of these embryos immediately after
hardening, since if kept for long in the absolute alcohol the osmic acid
specimens are apt to become brittle.
LIST OF WORKS REFERRED TO.
Gegenbaur. _Anat. der Wirbelthiere_, III Heft, Leipzig, 1873.
A. Götte. _Archiv. für Micr. Anat._, Vol. X. 1873. "Der Keim der
Forelleneies," _Archiv. für. Micr. Anat._, Vol. IX. 1873. "Untersuchung
über die Entwicklung der Bombinator igneus," _Archiv. für Micr. Anat._,
Vol. V. 1869. "Kurze Mittheilungen aus der Entwicklungsgeschichte der
Unke," _Archiv. für Micr. Anat._, Vol. IX. 1873.
Kupffer. _Archiv. für Micr. Anat._, Vol. II. 1866, p. 473. Ibid. Vol. IV.
1868, p. 209.
Kowalevsky. "Entwicklungsgeschichte der Holothurien," _Mémoires de l'Acad.
Impér. des Sciences de St Petersbourg_, vii ser. Vol. XI. 1867.
Kowalevsky, Owsjannikow, und Wagner. "Entwicklung der Störe," _Bulletin der
K. Acad. St Petersbourg_, Vol. XIV. 1873.
Kowalevsky. "Embryologische Studien an Würmern und Arthropoden," _Mémoirs
de l'Acad. Impér. des Sciences de St Petersbourg_, Vol. XVI. 1871.
E. Ray Lankester. _Annals and Mag. of Nat. History_, Vol. XI. 1873, p. 81.
W. Müller. "Ueber die Persistenz der Urniere bei Myxine Glutinosa,"
_Jenaische Zeitschrift_, Vol. VII. 1873.
Oellacher. _Zeitschrift für Wiss. Zoologie_, Vol. XXIII. 1873.
Owsjannikow. "Entwicklung der Coregonus," _Bul. der K. Akad. St
Petersbourg_, Vol. XIX.
Romiti. _Archiv. für Micr. Anat._, Vol. IX. 1873.
Waldeyer. _Eierstock u. Eie._
EXPLANATION OF PLATES 3 AND 4.
COMPLETE LIST OF REFERENCE LETTERS.
_al._ Alimentary canal. _ao._ Dorsal aorta. _auv._ Auditory vesicle. _bd._
Formative cell probably derived from the yolk. _cav._ Cardinal vein. _ch._
Notochord. _ch´._ Thickening of hypoblast to form the notochord. _eb._ Line
indicating the edge of the blastoderm. _ep._ Epiblast. _ep´._ Epidermis.
_er._ Embryonic rim. _es._ Embryonic swelling. _gl._ Glosso-pharyngeal
nerve. _h._ Head. _ht._ Heart. _hy._ Hypoblast. _ll._ Lower layer cells.
_ly._ Line of separation between the blastoderm and the yolk. _m._
Mesoblast. _mc._ Medullary canal. _mg._ Medullary groove. _mp._
Muscle-plate. _mp´._ Early formed mass of muscles. _n._ Peculiar nuclei
formed in the yolk. _n´._ Similar nuclei in the cells of the blastoderm.
_na._ Cells which help to close in the alimentary canal, and which are
derived from the yolk. _ny._ Network of lines present in the food-yolk.
_ol._ Olfactory pit. _op._ Eye. _ov._ Oviduct. _pn._ Pineal gland. _pov._
Projection which becomes the ovary. _pp._ Pleuro-peritoneal cavity. _pp´._
Remains of pleuro-peritoneal cavity in the head. _prv._ Protovertebræ.
_pwd._ Primary points of involution from the pleuro-peritoneal cavity by
the coalescence of which the Wolffian duct is formed. _sg._ Segmentation
cavity. _so._ Somatopleure. _sos._ Stalk connecting embryo with yolk-sac.
_sp._ Splanchnopleure. _spn._ Spinal nerve. _sur._ Suprarenal body. _ts._
Caudal lobes. _v._ Blood-vessel. _vg._ Vagus nerve. V. Fifth nerve. VII.
Seventh nerve. _vc_, 1, 2, 3, &c. 1st, 2nd and 3rd &c. visceral clefts.
_vp._ Vertebral plates. _wd._ Wolffian duct. _x._ Peculiar body underlying
the notochord derived from the hypoblast. _yk._ Yolk spherules.
All the figures were drawn with the Camera Lucida.
Plate 3.
Fig. 1. Section parallel with the long axis of the embryo through a
blastoderm, in which the floor of the segmentation cavity (_sg_) is not yet
completely lined by cells. The roof of the segmentation cavity is broken.
(Magnified 60 diam.) The section is intended chiefly to illustrate the
distribution of nuclei (_n_) in the yolk under the blastoderm. One of the
chief points to be noticed in their distribution is the fact that they form
almost a complete layer under the floor of the segmentation cavity. This
probably indicates that the cells whose nuclei they become take some share
in forming the layer of cells which subsequently (vide fig. 4) forms the
floor of the cavity.
Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the
time of the first appearance of the medullary groove. (Magnified 300 diam.)
The specimen is taken from a portion of the blastoderm which will form part
of the embryo. It shews two large nuclei of the yolk (_n_) and the network
in the yolk between them; this network is seen to be closer around the
nuclei than in the intervening space. The specimen further shews that there
are no areas representing cells around the nuclei.
Fig. 3. Section parallel with the long axis of the embryo through a
blastoderm, in which the floor of the segmentation cavity is not yet
covered by a complete layer of cells. (Magnified 60 diam.)
It illustrates (1) the characters of the epiblast, (2) the embryonic
swelling (_es_), (3) the segmentation cavity (_sg_). It should have been
drawn upon the same scale as fig. 4; the line above it represents its true
length upon this scale.
Fig. 4. Longitudinal section through a blastoderm at the time of the first
appearance of the embryonic rim, and before the formation of the medullary
groove. (Magnified 45 diam.)
It illustrates (1) the embryonic rim, (2) the continuity of epiblast and
hypoblast at edge of this, (3) the continual differentiation of the lower
layer cells, to form, on the one hand, the hypoblast, which is continuous
with the epiblast, and on the other the mesoblast, between this and the
epiblast; (4) the segmentation cavity, whose floor of cells is now
completed.
N.B. The cells at the embryonic end of the blastoderm have been made rather
too large.
Fig. 5. Surface view of the blastoderm shortly after the appearance of the
medullary groove. To shew the relation of the embryo to the blastoderm.
Fig. 6_a_ and _b_. Two transverse sections of the same embryo, shortly
after the appearance of the medullary groove. (Magnified 96 diam.)
_a._ In the region of the groove. It shews (1) the two masses of mesoblast
on each side, and the deficiency of the mesoblast underneath the medullary
groove; (2) the commencement of the closing in of the alimentary canal
below, chiefly from cells (_na_) derived from the yolk.
_b._ Section in the region of the head where the medullary groove is
deficient, other points as above.
Fig. 7_a_ and _b_. Two transverse sections of an embryo about the age or
rather younger than that represented in fig. 5. (Magnified 96 diam.)
_a._ Section nearer the tail; it shews the thickening of the hypoblast to
form the notochord (_ch´_).
In _b_ the thickening has become completely separated from the hypoblast as
the notochord. In _a_ the epiblast and hypoblast are continuous at the edge
of the section, owing to the section passing through the embryonic rim.
Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (1) the
flattened head (_h_) where the medullary groove is deficient, (2) the
caudal lobes, with a groove between them; it also shews that at this point,
the medullary groove has become roofed over and converted into a canal.
Fig. 8_a_. Transverse section of fig. 8, passing through the line _a_.
(Magnified 90 diam.) The section shews (1) the absence of the medullary
groove in the head and the medullary folds turning down at this time
instead of upwards; (2) the presence of the pleuro-peritoneal cavity in the
head (_pp_); (3) the completely closed alimentary canal (_al_).
Fig. 8_b_. Transverse section of fig. 8, through the line _b_. (Magnified
90 diam.) It shews (1) the neural canal completely formed; (2) the
vertebral plates of mesoblast not yet split up into somatopleure and
splanchnopleure.
Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object
a little older than the embryo represented in fig. 8. (Magnified 20 diam.)
The internal anatomy has hardly altered, with the exception of the
medullary folds having closed over above the head and the whole embryo
having become more folded off from the germ.
The two caudal lobes, and the very marked groove between them, are seen at
_ts_. The front end of the notochord became indistinct, and I could not see
its exact termination. The epithelium of the alimentary canal (_al_) is
seen closely underlying the notochord and becoming continuous with the
epiblast at the hind end of the notochord.
The first visceral cleft (1_vc_) and eye (_op_) are just commencing to be
formed, and the cranial flexure has just appeared.
Fig. 10. Section through the dorsal region of an embryo somewhat older than
the one represented in fig. 9. (Magnified 96 diam.)
It shews (1) the formation by a pinching off from the top of the alimentary
canal of a peculiar body which underlies the notochord (_x_); (2) the
primitive extension of the pleuro-peritoneal cavity up to the top of the
vertebral plates.
Plate 4.
Fig. 11_a_, _b_, and _c_. Three sections closely following each other from
an embryo in which three visceral clefts are present; _a_ is the most
anterior of the three. (Magnified 96 diam.) In all of these the
muscle-plates are shewn at _mp_. They have become separated from the
lateral plates in _b_ and _c_, but are still continuous with them in _a_.
The early formed mass of muscles is also shewn in all the figures (_mp´_).
The figures further shew (1) the formation of the spinal nerves (_spn_) as
small bodies of cells closely applied to the upper and outer edge of the
neural canal.
(2) The commencing formation of the cells which form the axial skeleton
from the inner (splanchnopleuric) layer of the muscle-plate. Sections _b_
and _c_ are given more especially to shew the mode of formation of the
oviduct (_ov_).
In _b_ it is seen as a _solid knob (ov)_, arising from the point where the
somatopleure and splanchnopleure unite, and in _c_ (the section behind _b_)
as a _solid rod (ov)_ closely applied to the epiblast, which has grown
backwards from the knob seen in _b_.
N.B. In all three sections only one side is completed.
Fig. 12_a_ and _b_. Two transverse sections of an embryo just before the
appearance of the external gills. (Magnified 96 diam.)
In _a_ there is seen to be an involution on each side (_pwd_), while _b_ is
a section from the space between two involutions from the pleuro-peritoneal
cavity, so that the Wolffian duct (at first solid) (_wd_) is not connected
as in _a_ with the pleuro-peritoneal cavity. The further points shewn in
the sections are--
(1) The commencing formation of the spiral valve (_al_).
(2) The suprarenal body (_sur_).
(3) The oviduct (_ov_), which has acquired a lumen.
(4) The increase in length of the muscle-plates, the spinal nerves,
&c.
Fig. 13. Section through the dorsal region of an embryo in which the
external gills are of considerable length. (Magnified 40 diam.) The chief
points to be noticed:
(1) The formation of the Wolffian body by outgrowths from
the Wolffian duct (_wd_).
(2) One of the still continuing connections (primitive
involutions) between the Wolffian duct and the
pleuro-peritoneal cavity (_pwd_).
(3) The oviduct largely increased in size (_ov_).
N.B. On the left side the oviduct has been accidentally made
too small.
(4) The growth downwards of the muscle-plate to form the
muscles of the abdomen.
(5) The formation of an outgrowth on each side of the
mesentery (_pov_), which will become the ovary.
(6) The spiral valve (_al_).
Fig. 14. Transparent view of the head of an embryo shortly before the
appearance of the external gills. (Magnified 20 diam.) The chief points to
be noticed are--
(1) The relation of the cranial nerves to the visceral
clefts and the manner in which the glosso-pharyngeal (_gl_)
and vagus (_vg_) are united.
(2) The remnants of the pleuro-peritoneal cavity in the head
(_pp_).
(3) The eye (_op_). The stalk, as well as the bulb of the
eye, are supposed to be in focus, so that the whole eye has
a somewhat peculiar appearance.
VI. A COMPARISON OF THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES[19].
Footnote 19: From the _Quarterly Journal of Microscopical
Science_, Vol. XV. 1875.
With Plate 5.
If the genealogical relationships of animals are to be mainly or largely
determined on embryological evidence, it becomes a matter of great
importance to know how far evidence of this kind is trustworthy.
The dependence to be placed on it has been generally assumed to be nearly
complete. Yet there appears to be no _à priori_ reason why natural
selection should not act during the embryonic as well as the adult period
of life; and there is no question that during their embryonic existence
animals are more susceptible to external forces than after they have become
full grown: indeed, an immense mass of evidence could be brought to shew
that these forces do act upon embryos, and produce in them great
alterations tending to obscure the genealogical inferences to be gathered
from their developmental histories. Even the time-honoured layers form to
this no exception. In _Elasmobranchii_, for instance, we find the notochord
derived from the hypoblast and the spinal ganglia derived from the
involuted epiblast of the neural canal, whilst in the higher vertebrates
both of these organs are formed in the mesoblast. Such instances are
leading embryologists to recognise the fact that the so-called layers are
not quite constant and must not be absolutely depended upon in the
determination of homologies. But though it is necessary to recognise the
fact that great changes do occur in animals during their embryonic life, it
is not necessary to conclude that all embryological evidence is thereby
vitiated; but rather it becomes incumbent on us to attempt to determine
which embryological features are ancestral and which secondary. For this
purpose it is requisite to ascertain what are the general characters of
secondary features and how they are produced. Many vertebrates have in the
first stages of their development a number of secondary characters which
are due to the presence of food material in the ovum; the present essay is
mainly an attempt to indicate how those secondary characters arose and to
trace their gradual development. At the same time certain important
ancestral characters of the early phases of the development of vertebrates,
especially with reference to the formation of the hypoblast and mesoblast,
are pointed out and their meaning discussed.
There are three orders of vertebrates of which no mention has been made,
viz., the _Mammals_, the _Osseous_ fishes, and the _Reptiles_. The first of
these have been passed over because the accounts of their development are
not sufficiently satisfactory, though as far as can be gathered from
Bischoff's account of the dog and rabbit there would be no difficulty in
shewing their relations with other vertebrates.
We also require further investigations on Osseous fishes, but it seems
probable that they develop in nearly the same manner as the Elasmobranchii.
With reference to Reptiles we have no satisfactory investigations.
* * * * *
Amphioxus is the vertebrate whose mode of development in its earliest
stages is simplest, and the modes of development of other vertebrates are
to be looked upon as modifications of this due to the presence of food
material in their ova. It is not necessary to conclude from this that
Amphioxus was the ancestor of our present vertebrates, but merely that the
earliest stages of development of this vertebrate ancestor were similar to
those of Amphioxus.
The ovum of Amphioxus contains very little food material and its
segmentation is quite uniform. The result of segmentation is a vesicle
whose wall is formed of a single layer of cells. These are all of the same
character, and the cavity of the vesicle called the segmentation cavity is
of considerable size. A section of the embryo, as we may now call the ovum,
is represented in Plate 5, fig. A I.
The first change which occurs is the pushing in of one half of the wall of
the vesicle towards the opposite half. At the same time by the narrowing of
its mouth the hollow hemisphere so formed becomes again a vesicle[20].
Footnote 20: I have been able to make at Naples observations
which confirm the account of the invagination of Amphioxus as
given by Kowalevsky, though my observations are not nearly so
complete as those of the Russian naturalist.
Owing to its mode of formation the wall of this secondary vesicle is
composed of two layers which are only separated by a narrow space, the
remnant of the segmentation cavity.
Two of the stages in the formation of the secondary vesicle by this process
of involution are shewn in Plate X, fig. A II, and A III. In the second of
these the general growth has been very considerable, rendering the whole
animal much larger than before. The cavity of this vesicle, A III, is that
of the commencing alimentary canal whose final form is due to changes of
shape undergone by this primitive cavity. The inner wall of the vesicle
becomes converted into the wall of the alimentary canal or hypoblast, and
also into part or the whole of the mesoblast.
During the involution the cells which are being involuted undergo a change
of form, and before the completion of the process have acquired a
completely different character to the cells forming the external wall of
the secondary vesicle or epiblast. This change of character in the cells is
already well marked in fig. A II. It is of great importance, since we shall
find that some of the departures from this simple mode of development,
which characterise other vertebrates, are in part due to the distinction
between the hypoblast and epiblast cells appearing during segmentation, and
not subsequently as in Amphioxus during the involution of the hypoblast.
Kowalevsky (_Entwicklungsgeschichte des Amphioxus_) originally believed
that the narrow mouth of the vesicle (according to Mr Lankester's
terminology _blastopore_) became the anus of the adult. He has since, and
certainly correctly, given up this view. The opening of the involution
becomes closed up and the adult anus is no doubt formed as in all other
vertebrates by a pushing in from the exterior, though it probably
corresponds in position very closely with the point of closing up of the
original involution.
The mode of formation of the mesoblast is not certainly known in Amphioxus;
we shall find, however, that for all other vertebrates it arises from the
cells which are homologous with the involuted cells of this animal.
Since food material is a term which will be very often employed, it will be
well to explain exactly the sense in which it will be used. It will be used
only with reference to those passive highly refractive particles which are
found embedded in most ova.
In some eggs, of which the hen's egg may be taken as a familiar example,
the yolk-spherules or food material form the larger portion of the ovum,
and a distinction is frequently made between the germinal disc and the
yolk.
This distinction is, however, apt to lead to a misconception of the true
nature of the egg. There are strong grounds for believing that the
so-called yolk, equally with the germinal disc, is composed of an active
protoplasmic basis endowed with the power of growth, in which passive
yolk-spherules are embedded; but that the part ordinarily called the yolk
contains such a preponderating amount of yolk-spherules that the active
basis escapes detection, and does not exhibit the same power of growth as
the germinal disc.
With the exception of mammals, whose development requires to be more
completely investigated, Amphioxus is as far as we know the only vertebrate
whose ovum does not contain a large amount of food material.
In none of these (vertebrate) yolk-containing ova is the food material
distributed uniformly. It is always concentrated much more at one pole than
at the other, and the pole at which it is most concentrated may be
conveniently called the lower pole of the egg.
In eggs in which the distribution of food material is not uniform
segmentation does not take place with equal rapidity through all parts of
the egg, but its rapidity is, roughly speaking, inversely proportional to
the quantity of food material.
When the quantity of food material in a part of the egg becomes very great,
segmentation does not occur at all; and even in those cases where the
quantity of food yolk is not too great to prevent segmentation the
resulting segmentation spheres are much larger than where the yolk-granules
are more sparsely scattered.
The Frog is the vertebrate whose development comes nearest to that of
Amphioxus, as far as the points we are at present considering are
concerned. But it will perhaps facilitate the understanding of their
relations shortly to explain the diagrammatic sections which I have given
of an animal supposed to be intermediate in its development between the
Frog and Amphioxus. Plate 5, fig. B I, represents a longitudinal section of
this hypothetical egg at the close of segmentation. The lower pole,
coloured yellow, represents the part containing more yolk material, and the
upper pole, coloured blue, that with less yolk. Owing to the presence of
this yolk the lower pole even at the close of segmentation is composed of
cells of a different character to those of the upper pole. In this respect
this egg can already be distinguished from that of Amphioxus, in which no
such difference between the two poles is apparent at the corresponding
period (Plate 5, fig. A I).
The segmentation cavity in this ovum is not quite so large proportionately
as in Amphioxus, and the encroachment upon it is due to the larger bulk of
the lower pole of the egg. In fig. B II the involution of the lower pole
has already commenced; this involution is (1) not quite symmetrical, and
(2) on the ventral side (the left side) the epiblast cells forming the
upper part of the egg are growing round the cells of the lower pole of the
egg or lower layer cells. Both of these peculiarities are founded upon what
happens in the Frog and the Selachian, but it is to be noticed that the
change from the lower layer cells being involuted towards the epiblast
cells, to the epiblast cells growing round the lower layer cells, is a
necessary consequence of the increased bulk of the latter.
In this involution not only are the cells of the lower pole pushed on, but
also some of those of the upper or yellow portion; so that in this as in
all other cases the true distinction between the epiblast and hypoblast
does not appear till the involution to form the latter is completed. In the
next stage, B III, the involution has become nearly completed and the
opening to the exterior or blastopore quite constricted.
The segmentation cavity has been entirely obliterated, as would have been
found to be the case with Amphioxus had the stage a little older than that
on Plate 5, A III, been represented. The cavity marked (_al_), as was the
case with Amphioxus, is that of the alimentary canal.
The similarities between the mode of formation of the hypoblast and
alimentary canal in this animal and in Amphioxus are so striking and the
differences between the two cases so slight that no further elucidation is
required. One or two points need to be spoken of in order to illustrate
what occurs in the Frog. When the involution to form the alimentary canal
occurs, certain of the lower layer cells (marked _hy_) become distinguished
from the remainder of the lower layer cells as a separate layer and form
the hypoblast which lines the alimentary canal. It is to be noticed that
the cells which form the ventral epithelium of the alimentary canal are not
so soon to be distinguished from the other lower layer cells as those which
form its dorsal epithelium. This is probably a consequence of the more
active growth, indicated by the asymmetry of the involution, on the dorsal
side, and is a fact with important bearings in the ova with more food
material. The cells marked _m_ and coloured red also become distinguished
as a separate layer from the remainder of the hypoblast and form the
mesoblast. The remainder of the lower layer cells form a mass equivalent to
the yolk-sac of many vertebrates, and are not converted directly into the
tissues of the animal.
Another point to be noticed is the different relation of epiblast cells to
the hypoblast cells at the upper and lower side of the mouth of the
involution. Above it, on its dorsal side, the epiblast and hypoblast are
continuous with one another. On its ventral side they are primitively not
so continuous. This is due to the epiblast, as was before mentioned,
growing round the lower layer cells on the ventral side, vide B II, and
merely remaining continuous with them on the dorsal. The importance of
these two points will appear when we come to speak of other vertebrates.
The next animal whose development it is necessary to speak of is the Frog,
and its differences from the mode of development are quite easy to follow
and interpret. Segmentation is again not uniform, and results in the
formation of an upper layer of smaller cells and a lower one of larger; in
the centre is a segmentation cavity. The stage at the close of segmentation
is represented in C I. From the diagram it is apparent that the lower layer
cells occupy a larger bulk than they did in the previous animal (Plate 5, B
I), and tend to encroach still more upon the segmentation cavity, otherwise
the differences between the two are unimportant. There are, however, two
points to be noted. In the first place, although the cells of the upper
pole are distinguished in the diagrams from the lower by their colour, it
is not possible at this stage to say what will become epiblast and what
hypoblast. In the second place the cells of the upper pole or epiblast
consist of two layers--an outer called the epidermic layer and an inner
called the nervous. In the previous cases the epiblast consisted of a
single layer of cells. The presence of these two layers is due to a
distinction which, arising in most other vertebrates late, in the Frog
arises early. In most other vertebrates in the later stages of development
the epiblast consists of an outer layer of passive and an inner of active
cells. In the Frog and other Batrachians these two layers become
distinguished at the commencement of development.
In the next stage (C II) we find that the involution to form the alimentary
canal has commenced (_al_), but that it is of a very different character to
the involution in the previous case. It consists in the growing inwards of
a number of cells from the point _x_ (C I) towards the segmentation cavity.
The cells which grow in this way are partly the blue cells and partly the
smaller yellow ones. At first this involuted layer of cells is only
separated by a slit from the remainder of the lower layer cells; but by the
stage represented in C II this has widened into an elongated cavity (_al_).
In its formation this involution pushes backwards the segmentation cavity,
which finally disappears in the stage C III. The point _x_ remains
practically stationary, but by the general growth of the epiblast,
mesoblast and hypoblast, becomes further removed from the segmentation
cavity in C II than in C I. On the opposite side of the embryo to that at
which the involution occurs the epiblast cells as before, grow round the
lower layer cells. The commencement of this is already apparent in C I, and
in C II the process is nearly completed, though there is still a small mass
of yolk filling up the blastopore. The features of this involution are in
the main exaggerations of what was supposed to occur in the previous
animal. The asymmetry of the involution is so great that it is completely
one-sided and results, in the first instance, in a mere slit; and the whole
process of enclosing the yolk by epiblast is effected by the epiblast cells
on the side of the egg opposite to the involution.
The true mesoblast and hypoblast are formed precisely as in the previous
case. The involuted cells become separated into two layers, one forming the
dorsal epithelium of the alimentary canal, and a layer between this and the
epiblast forming the mesoblast. There is also a layer of mesoblast
accompanying the epiblast which encloses the yolk, which is derived from
the smaller yellow cells at _y_ (C I). The edge of this mesoblast, _m´_,
forms a thickened ridge, a feature which persists in other vertebrates.
It is a point of some importance for understanding the relation between the
mode of formation of the alimentary canal in the Frog and other vertebrates
to notice that on the ventral surface the cells which are to form the
epithelium of the alimentary canal become distinguished as such very much
later than do those to form its dorsal epithelium, and are derived not from
the involuted cells but from the primitive large yolk-cells. It is indeed
probable that only a very small portion of epithelium of the ventral wall
of the mid-gut is in the end derived from these larger yolk-cells. The
remainder of the yolk-cells (C III, and C II, _yk_) form the yolk mass and
do not become directly formed into the tissues of the animal.
In the last stage I have represented for the frog, C III, there are several
features to be noticed.
The direct connection at their hind-ends between the cavities of the neural
and alimentary canals is the most important of these. This is a result of
the previous continuity of the epiblast and hypoblast at the point _x_, and
is a feature almost certainly found in Amphioxus, but which I will speak of
more fully in my account of the Selachian's development. The opening of the
blastopore called the anus of Rusconi is now quite narrowed, it does not
become the anus of the adult. It may be noticed that at the front end of
the embryo the primitive dorsal epithelium of the alimentary canal is
growing in such a way as to form the epithelium both of the dorsal and
ventral surfaces of the fore-gut.
In spite of various features rendering the development of the Frog more
difficult of comprehension than that of most other vertebrates, it is easy
to see that the step between it and Amphioxus is not a very great one, and
will very likely be bridged over at some future time, when our knowledge of
the development of other forms becomes greater.
From the Frog to the Selachian is a considerable step, but I have again
hypothetically sketched a type intermediate between them whose development
agrees in some important points with that of _Pelobates fuscus_ as
described by Bambeke. The points of agreement, though not obvious at first
sight, I shall point out in the course of my description.
The first stage (D I), at the close of segmentation, deserves careful
attention. The segmentation cavity by the increase of the food yolk is very
much diminished in size, and, what is still more important, has as it were
sunk down so as to be completely within the _lower layer cells_. The roof
of the segmentation cavity is thus formed of epiblast and lower layer
cells, a feature which Bambeke finds in _Pelobates fuscus_ and which is
certainly found in the Selachians. In the Frog we found that the
segmentation cavity began to be encroached on by the lower layer cells, and
from this it is only a small step to find these cells creeping still
further up and forming the roof of the cavity. In the lower layer cells
themselves we find an important new feature, viz. that during segmentation
they become divided in two distinct parts--one of these where the segments
owing to the presence of much food yolk are very large, and the other where
the segments are much smaller.
The separation between these two is rather sharp. Even this separation was
foreshadowed in the Frog's egg, in which a number of lower layer cells were
much smaller and more active at the two sides of the segmentation cavity
than elsewhere. The segmentation cavity at first lies completely within the
region of the small spheres. The larger cells serve almost entirely as food
yolk. The epiblast, as is normal with vertebrates, consists of a single
layer of columnar cells.
In the next stage (D II) the formation of the alimentary canal (_al_) has
commenced, but it is to be observed that there is in this case _no true
involution_.
As an accompaniment to the encroachment upon the segmentation cavity, which
was a feature of the last stage, the cells to form the walls of the
alimentary canal have come to occupy their final position during
segmentation and without the intermediation of an involution, and traces
only of the involution, are to be found in (1) a split in the lower layer
cells which passes along the line separating the small and the large lower
layer cells; and (2) in the epiblast becoming continuous with the hypoblast
on the dorsal side of the mouth of this split. It is even possible that at
this point a few cells (though certainly only a very small number) of those
marked blue in D I become involuted. This point in this, as in all other
cases, is the tail end of the embryo. The other features of this stage are
as follows:--(1) The segmentation cavity has become smaller and less
conspicuous than it was. (2) The epiblast cells have begun to grow round
the yolk even in a more conspicuous manner than they did in the Frog, and
are accompanied by a layer of mesoblast cells which again becomes thickened
at its edge. The mesoblast cells in the region of the body are formed in
the same way as before, viz. by the separation of a layer to form the
epithelium of the alimentary canal, the other cells remaining as mesoblast;
and as in the Frog, or in a more conspicuous manner, we find that the
dorsal surface only of the alimentary cavity has a wall formed of a
_distinct layer of cells_, but on the ventral side the cavity is at first
closed in by the large spheres of the yolk only. The formation of the
alimentary canal by a split and not by an involution is exactly what
Bambeke finds in _Pelobates_.
The next stage, D III, is about an equivalent age to C III in the Frog. It
exhibits the same connection between the neural and the alimentary canals
as was found there.
The alimentary canal is beginning to become closed in below, and this
occurs near the two ends earlier than in the middle. The cells to form the
ventral wall are derived from the large yolk-cells. The non-formation of
the ventral wall of the alimentary canal so soon in the middle as at the
ends is an early trace of the umbilical canal found in Birds and
Selachians, by which the alimentary tract is placed in communication with
the yolk-sac. The segmentation cavity has by this stage completely
vanished, and the epiblast with its accompanying mesoblast has spread
completely round the yolk material so as to form the ventral wall of the
body.
Though in some points this manner of development may seem to differ from
that of the Frog, there is really a fundamental agreement between the two,
and between this mode of development and that of the Selachians we shall
find the agreement to be very close.
After segmentation we find that the egg of a Selachian consists of two
parts--one of these called the germinal disc or blastoderm, and the other
the yolk. The former of these corresponds with the epiblast and the part of
the lower pole composed of smaller segments in the last-described egg, and
the latter to the larger segments of the lower pole. This latter division,
owing to the quantity of _yolk_ which it contains, has not undergone
segmentation, but its homology with the larger segments of the previous
eggs is proved (1) by its containing a number of nuclei (E I, _n_), which
become the nuclei of true cells and enter the blastoderm, and (2) by the
presence in it of a number of lines forming a network similar to that of
many cells. The segmentation cavity, as before, lies completely within the
lower layer cells.
The next stage, E II, is almost precisely similar to the second stage of
the last egg. As there, the primitive involution is merely represented by a
split separating the yolk and the germinal disc, and on the dorsal side
alone is there a true cellular wall for this split, and at the dorsal mouth
of the split the alimentary epithelium becomes continuous with the
epiblast.
The segmentation cavity has become diminished, and round the yolk the
epiblast, accompanied by a layer of mesoblast, is commencing to grow. In
this growth all parts of the blastoderm take a share except that part where
the epiblast and hypoblast are continuous. This manner of growth is
precisely what occurs in the Frog, though there it is not so easily made
out; and not all the investigators who have studied the Frog have
understood the exact meaning of the appearances they have seen and drawn.
This similarity of relation of the epiblast to the yolk in the two cases is
a further confirmation of the identity of the Selachian's yolk with the
large yolk-spheres of the previous eggs.
The next stage, E III, is in many ways identical with the corresponding
stage in the last-described egg, and in the same way as in that case the
neural and alimentary canals are placed in communication with each other.
The mode in which this occurs will be easily gathered from a comparison of
E II and E III. It is the same for the Selachians and Batrachians. The
neural canal (_nc_) is by the stage figured E III, completely formed in the
way so well known in the Bird, and between the roof of the canal and the
external epiblast a layer of mesoblast has already grown in. The floor of
the neural canal is the same layer marked _ep_ in E II, and therefore
remains continuous with the hypoblast at _x_; and when by a simultaneous
process the roof of the neural canal and the ventral wall of the alimentary
become formed by the folding over of one continuous layer (the epiblast and
hypoblast continuous at the point _x_), the two canals, viz. the neural and
alimentary, are necessarily placed in communication at their hind-ends, as
is seen in the diagram.
There are several important points of difference between E III and D III.
In the first place, owing to the larger size of the yolk mass in E III, the
epiblast, accompanied by mesoblast, has not proceeded nearly so far round
it as in the previous case. It is also worth notice that at the right as
well as at the left end of the germinal disc the epiblast is commencing to
grow round the yolk. The yolk has, however, become surrounded to a much
smaller extent on the right hand than on the left. Since, in the earlier
stage, the epiblast became continuous with the hypoblast at _x_, it is not
from sections obvious how this occurs. I have therefore appended a diagram
to explain it (E´). The blastoderm rests like a disc on the yolk and grows
over it on all sides, except at the point where the epiblast and hypoblast
are continuous (_x_). This point becomes as it were left in a bay. Next the
two sides of the bay coalesce, the bay becomes obliterated, and the effect
produced is exactly as if the blastoderm had grown round the yolk at the
point _x_ (corresponding with the tail of the embryo) as well as everywhere
else. It thus comes about that the final point where the various parts of
the blastoderm meet and completely enclose the yolk mass does not
correspond with the anus of Rusconi of the Frog, but is at some little
distance from the hind-end of the embryo. In other words, the position of
the blastopore in the Selachian is not the same as in the Frog.
Another point deserving attention is the formation of the ventral wall of
the alimentary canal. This takes place in two ways--partly by a folding-in
at the sides and end, and partly from cells formed around the nuclei (_n_)
in the yolk. From these a large portion of the ventral wall of the mid-gut
is formed.
The folding-in of the sheet of hypoblast to assist in the closing-in of the
ventral wall of the alimentary canal is a consequence of the flattened form
of the original alimentary slit which is far too wide to form the cavity of
the final canal. In the Bird whose development must next be considered this
folding-in is a still more prominent feature in the formation of the
alimentary canal. As in the last case, the alimentary canal is widely open
in the middle to the yolk at the time when its two ends are closed below
and shut off from it; still later this opening becomes very narrow and
forms the duct of the so-called umbilical cord which places the yolk-sac in
communication with the alimentary canal. As the young animal becomes larger
the yolk-sac ceases to communicate directly with the alimentary canal, and
is carried about by it for some time as an appendage and only at a later
period shrivels up.
The mesoblast is formed in a somewhat different way in the Sharks than in
other vertebrates. It becomes split off from the hypoblast, not in the form
of a single sheet as in other vertebrates, but as two lateral sheets, one
on each side of the middle line and separated from one another by a
considerable interval; whilst the notochord is derived not as in other
vertebrates from the mesoblast, but from the hypoblast (vide F. M. Balfour,
"Development of Selachians[21]," _Journal of Microscopical Science_, Oct.,
1874).
Footnote 21: Paper No. V, p. 82 _et seq._ in this edition.
Between the Selachians and the Aves there is a considerable gulf, which it
is more difficult satisfactorily to bridge over than in the previous cases;
owing to this I have not attempted to give any intermediate stage between
them.
The first stage of the Bird (F I) is very similar in many respects to the
corresponding stage in the Selachian. The segmentation cavity is, however,
a less well-defined formation, and it may even be doubted whether a true
segmentation cavity, homologous with the segmentation cavity in the
previously described eggs, is present. On the floor of the cavity which is
formed by the yolk are a few larger cells known as formative cells which,
according to Götte's observations, are derived from the yolk, in a somewhat
similar manner to the cells which were formed around the nuclei in the
Selachian egg, and which helped to form the ventral wall of the alimentary
canal. Another point to be noticed is that the segmentation cavity occupies
a central position, and not one to the side as in the Selachian.
The yolk is proportionately quite as large as in the Selachian's egg, but,
as in that case, there can be little or no doubt of its being homologous
with the largest of the segmentation spheres of the previous eggs. It does
not undergo segmentation. The epiblast is composed of columnar cells, and
extends a short way beyond the edge of the lower layer cells.
In the next stage the more important departures from the previous type of
development become visible.
The epiblast spreads uniformly over the yolk-sac and not on the one side
only as in the former eggs.
This is due to the embryo (indicated in F II by a thickening of the cells)
lying in the centre and not at the edge of the blastoderm. A necessary
consequence of this is, that the epiblast does not, as in the previous
cases, become continuous with the hypoblast at the tail end of the embryo.
This continuity, being of no functional importance, could easily be
dispensed with, and the central position of the embryo may perhaps be
explained by supposing the process, by which in the Selachian egg the
blastopore ceases to correspond in position with the opening of the
alimentary slit or anus of Rusconi (vide E´), to occur quite early during
segmentation instead of at a late period of development. For the
possibility of such a change in the date of formation, the early appearance
of the nervous and epidermic layers in the Frog affords a parallel.
The epiblast in its growth round the yolk is only partially accompanied by
mesoblast, which, however, is thickened at its extreme edge as in the Frog.
Owing to the epiblast not becoming continuous with the hypoblast at the
tail end of the embryo, the alimentary slit is not open to the exterior.
The hypoblast is formed by some of the lower layer cells becoming
distinguished as a separate layer; the remainder of the lower layer cells
become the mesoblast.
The formation of the mesoblast and hypoblast out of the lower layer cells
has been accepted for the Bird by most observers, but has been disputed by
several, and recently by Kölliker. These have supposed that the mesoblast
is derived from the epiblast. I feel convinced that these observers are in
the wrong, and that the mesoblast is genuinely derived from the lower layer
cells.
The greater portion of the alimentary cavity consists of the original
segmentation cavity (vide diagrams). This feature of the segmentation
cavity of Birds sharply distinguishes it from any segmentation cavity of
other eggs, and renders it very doubtful whether the similarly named
cavities of the Bird and of other vertebrates are homologous. On the floor
of the cavity are still to be seen some of the formative cells, but
observers have not hitherto found that they take any share in forming the
ventral wall of the alimentary canal.
The features of the next stage are the necessary consequences of those of
the last.
The ventral wall of the alimentary canal is entirely formed by a folding-in
of the sheet of hypoblast.
The more rapid folding-in at the head still indicates the previous more
vigorous growth there, otherwise there is very little difference between
the forms of the fold at the head and tail. The alimentary canal does not
of course, at this or any period, communicate with the neural tube, since
the epiblast and hypoblast are never continuous. The other features, such
as the growth of the epiblast round the yolk-sac, are merely continuations
of what took place in the last stage.
In the development of a yolk-sac as a distinct appendage, and its
absorption within the body, at a later period, the bird fundamentally
resembles the Dog-fish.
Although there are some difficulties in deriving the type of development
exhibited by the Bird directly from that of the Selachian, it is not very
difficult to do so directly from Amphioxus. Were the alimentary involution
to remain symmetrical as in Amphioxus, and the yolk-containing part of the
egg to assume the proportions it does in the Bird, we should obtain a mode
of development which would not be very dissimilar to that of the Bird. The
epiblast would necessarily overgrow the yolk uniformly on all sides and not
in the unsymmetrical fashion of the Selachian egg. A confirmation of this
view might perhaps be sought for in the complete difference between the
types of circulation of the yolk-sac in Birds and Selachians; but this is
not so important as might at first sight appear, since it is not from the
Selachian egg but from some Batrachian that it would be necessary to derive
the Reptiles' and Birds' eggs.
If this view of the Bird's egg be correct, we are compelled to suppose that
the line of ancestors of Birds and Reptiles did not include amongst them
the Selachians and the Batrachians, or at any rate Selachians and
Batrachians which develop on the type we now find.
The careful investigation of the development of some Reptiles might very
probably throw light upon this important point. In the meantime it is
better to assume that the type of development of Birds is to be derived
from that of the Frog and Selachians.
_Summary._--If the views expressed in this paper are correct, all the modes
of development found in the higher vertebrates are to be looked upon as
modifications of that of Amphioxus. It is, however, rather an interesting
question whether it is possible to suppose that the original type was _not_
that of Amphioxus, but of some other animal, say, for instance, that of the
Frog, and that this varied in two directions,--on the one hand towards
Amphioxus, in the reverse direction to the course of variation presupposed
in the text; and on the other hand in the direction towards the Selachians
as before.
The answer to this question must in my opinion be in the negative. It is
quite easy to conceive the food material of the Frog's egg completely
vanishing, but although this would entail simplifications of development
and possibly even make segmentation uniform, there would, as far as I can
see, be no cause why the essential features of difference between the
Frog's mode of development and that of Amphioxus should change. The
asymmetrical and slit-like form of involution on the one side and the
growth of the epiblast over the mesoblast on the other side, both
characteristics of the present Frog's egg, would still be features in the
development of the simplified egg.
In the Mammal's egg we probably have an example of a Reptile's egg
simplified by the disappearance of the food material; and when we know more
of Mammalian embryology it will be very interesting to trace out the exact
manner in which this simplification has affected the development. It is
also probable that the eggs of Osseous fish are fundamentally simplified
Selachian eggs; in which case we already know that the diminution of food
material has affected but very slightly the fundamental features of
development.
One common feature which appears prominently in reviewing the embryology of
vertebrates as a whole is the derivation of the mesoblast from the
hypoblast; in other words, we find that it is from the layer corresponding
to that which becomes involuted in Amphioxus so as to line the alimentary
cavity that the mesoblast is split off.
That neither the hypoblast or mesoblast can in any sense be said to be
derived from the epiblast is perfectly clear. When the egg of Amphioxus is
in the blastosphere stage we cannot speak of either an epiblast or
hypoblast. It is not till the involution or what is equivalent has
occurred, converting the single-walled vesicle into a double-walled one,
that we can speak of these two layers. It might seem scarcely necessary to
insist upon this point, so clear is it without explanation, were it not
that certain embryologists have made a confusion about it.
The derivation of the mesoblast from the hypoblast is the more interesting,
since it is not confined to the vertebrates, but has a very wide extension
amongst the invertebrates. In the cases (whose importance has been recently
insisted upon by Professor Huxley), of the Asteroids, the Echinoids,
Sagitta, and others, in which the body-cavity arises as an outgrowth of the
alimentary canal and the somatopleure and splanchnopleure are formed from
that outgrowth, it is clear without further remark that the mesoblast is
derived from the hypoblast. For the Echinoderms in which the water-vascular
system and muscular system arise as a solid outgrowth of the wall of the
alimentary canal there can also be no question as to the derivation of the
mesoblast from the hypoblast.
Amongst other worms, in addition to Sagitta, the investigations of
Kowalevsky seem to shew that in Lumbricus the mesoblast is derived from the
hypoblast.
Amongst Crustaceans, Bobretsky's[22] observations on Oniscus (_Zeitschrift
für wiss. Zoologie_, 1874) lead to the same conclusion.
Footnote 22: He says, p. 182: "Bevor aber die Hälfte der
Eioberfläche von den Embryonalzellen bedeckt ist, kommt die
erste gemeinsame Anlage des mittleren und unteren Keimblattes
zum Vorschein."
In Insects Kowalevsky's observations lead to the conclusion that mesoblast
and hypoblast arise from a common mass of cells; Ulianin's observations
bring out the same result for the abnormal Poduridæ, and Metschnikoff's
observations shew that this also holds for Myriapods.
In Molluscs the point is not so clear.
In Tunicates, even if we are not to include them amongst vertebrates[23],
the derivation of mesoblast from hypoblast is without doubt.
Footnote 23: Anton Dohrn, _Der Ursprung des Wirbelthieres_.
Leipzig, 1875.
Without going further into details it is quite clear that the derivation of
the mesoblast from the hypoblast is very general amongst invertebrates.
It will hardly be disputed that primitively the muscular system of the
body-wall could not have been derived from the layer of cells which lines
the alimentary canal. We see indeed in Hydra and the Hydrozoa that in its
primitive differentiation, as could have been anticipated beforehand, the
muscular system of the body is derived from the epiblast cells. What, then,
is the explanation of the widespread derivation of the mesoblast, including
the muscular system of the body, from the hypoblast?
The explanation of it may, I think, possibly be found, and at all events
the suggestion seems to me sufficiently plausible to be worth making, in
the fact that in many cases, and probably this applies to the ancestors of
the vertebrates, the body-cavity was primitively a part of the alimentary.
Mr Lankester, who has already entered into this line of speculation, even
suggests (_Q. J. of Micr. Science_, April, 1875) that this applies to all
higher animals. It might then be supposed that the muscular system of part
of the alimentary canal took the place of the primitive muscular system of
the body; so that the whole muscular system of higher animals would be
primitively part of the muscular system of the digestive tract.
I put this forward merely as a suggestion, in the truth of which I feel no
confidence, but which may perhaps induce embryologists to turn their
attention to the point. If we accept it for the moment, the supplanting of
the body muscular system by that of the digestive tract may hypothetically
be supposed to have occurred in the following way.
When the diverticulum or rather paired diverticula were given off from the
alimentary canal they would naturally become attached to the body-wall, and
any contractions of their intrinsic muscles would tend to cause movements
in the body-wall. So far there is no difficulty, but there is a
physiological difficulty in explaining how it can have happened that this
secondary muscular system can have supplanted the original muscular system
of the body.
The following suggestions may lessen this difficulty, though perhaps they
hardly remove it completely. If we suppose that the animal in which these
diverticula appeared had a hard test and was not locomotive, the intrinsic
muscular system of the body would naturally completely atrophy. But since
the muscular system of the diverticula from the stomach would be required
to keep up the movement of the nutritive fluid, it would not atrophy, and
were the test subsequently to become soft and the animal locomotive, would
naturally form the muscular system of the body. Or even were the animal
locomotive in which the diverticula appeared, it is conceivable that the
two systems might at first coexist together; that either (1) subsequently
owing to the greater convenience of early development, the two systems
might acquire a development from the same mass of cells and those the cells
of the inner or hypoblast layer, so that the derivation of the body muscles
from the hypoblast would only be apparent and not real, or (2) owing to
their being better nourished as they would necessarily be, and to their
possibly easier adaptability to some new form of movement of the animal,
the muscle-cells of the alimentary canal might become developed exclusively
whilst the original muscular system atrophied.
I only hold this view provisionally till some better explanation is given
of the cases of Sagitta and the Echinoderms, as well as of the nearly
universal derivation of the mesoblast from the hypoblast. The cases of this
kind may be due to some merely embryonic changes and have no meaning in
reference to the adult condition, but I think that we have no right to
assume this till some explanation of the embryonic can be suggested.
For vertebrates, I have shewn that in Selachians the body-cavity at first
extends quite to the top of what becomes the muscle plate, so that the line
or space separating the two layers of the muscle plate (vide Balfour,
'Development of Elasmobranch Fishes[24],' _Quart. Journ. of Micro. Science_
for Oct., 1874. Plate XV, fig. 11,_a_, 11_b_, 12_a_, _mp_.) is a portion of
the original body-cavity. If this is a primitive condition, which is by no
means certain, we have a condition which we might expect, in which both the
inner and the outer wall of the primitive body-cavity assists in forming
the muscular system of the body.
Footnote 24: Paper No. V, p. 60 _et seq._ of this edition,
pl. 4, figs. 11_a_, 11_b_, 12_a_, _mp_.
It is very possible that the formation of the mesoblast as two masses, one
on each side of the middle line as occurs in Selachians, and which as I
pointed out in the paper quoted above also takes place in some worms, is a
remnant of the primitive formation of the body-cavity as paired outgrowth
of the alimentary canal. This would also explain the fact that in
Selachians the body-cavity consists at first of two separate portions, one
on each side of the alimentary canal, which only subsequently become united
below and converted into a single cavity (vide _loc. cit._[25], Plate XIV,
fig. 8_b_, _pp_).
Footnote 25: Pl. 3 of this edition, fig. 8_b_, _pp_.
In the Echinoderms we find instances where the body-cavity and
water-vascular system arise as an outgrowth from the alimentary canal,
which subsequently becomes constricted off from the latter (Asteroids and
Echinoids), together with other instances (Ophiura, Synapta) where the
water-vascular system and body-cavity are only secondarily formed in a
solid mass of mesoblast originally split off from the walls of the
alimentary canal.
These instances shew us how easily a change of this kind may take place,
and remove the difficulty of understanding why in vertebrates the
body-cavity never communicates with the alimentary.
The last point which I wish to call attention to is the blastopore or anus
of Rusconi.
This is the primitive opening by which the alimentary canal communicates
with the exterior, or, in other words, the opening of the alimentary
involution. It is a distinctly marked structure in Amphioxus and the
Batrachians, and is also found in a less well-marked form in the
Selachians; in Birds no trace of it is any longer to be seen. In all those
vertebrates in which it is present, it closes up and does not become the
anus of the adult. The final anus nevertheless corresponds very closely in
position with the anus of Rusconi. Mr Lankester has shewn (_Quart. Journ.
of Micro. Science_ for April, 1875) that in invertebrates as well as
vertebrates the blastopore almost invariably closes up. It nevertheless
corresponds as a rule very nearly in position either with the mouth or with
the anus.
If this opening is viewed, as is generally done, as really being the mouth
in some cases and the anus in others, it becomes very difficult to believe
that the blastopore can in all cases represent the same structure. In a
single branch of the animal kingdom it sometimes forms the mouth and
sometimes the anus: thus for instance in Lumbricus it is the mouth
(according to Kowalevsky), in Palæmon (Bobretzky) the anus. Is it credible
that the mouth and anus have become changed, the one for the other?
If, on the other hand, we accept the view that the blastopore never becomes
either the one or the other of these openings, it is, I think, possible to
account for its corresponding in position with the mouth in some cases or
the anus in others.
That it would soon come to correspond either with the mouth or anus
(probably with the earliest formed of these in the embryo), wherever it was
primitively situated, follows from the great simplification which would be
effected by its doing so. This simplification consists in the greater
facility with which the fresh opening of either mouth or anus could be made
where the epiblast and hypoblast were in continuity than elsewhere. Even a
change of correspondence from the position of the mouth to that of the anus
or _vice versa_ could occur. The mode in which this might happen is
exemplified by the case of the Selachians. I pointed out in the course of
this paper how the final point of envelopment of the yolk became altered in
Selachians so as to cease to correspond with the anus of Rusconi; in other
words, how the position of the blastopore became changed. In such a case,
if the yolk material again became diminished, the blastopore would
correspond in position with neither mouth nor anus, and the causes which
made it correspond in position with the anus before, would again operate,
and make it correspond in position perhaps with the mouth. Thus the
blastopore might absolutely cease to correspond in position with the anus
and come to correspond in position with the mouth.
It is hardly possible to help believing that the blastopore primitively
represented a mouth. It may perhaps have lost this function owing to an
increase of food yolk in the ovum preventing its being possible for the
blastopore to develop directly into a mouth, and necessitating the
formation of a fresh mouth. If such were the case, there would be no reason
why the blastopore should ever again serve functionally as a mouth in the
descendants of the animal which developed this fresh mouth.
EXPLANATION OF PLATE 5.
COMPLETE LIST OF REFERENCES.
_al._ Cavity of alimentary canal. _bl._ Blastoderm. _ch._ Notochord. _ep._
Epiblast. _em._ Embryo. _f._ Formative cells. _hy._ Hypoblast. _ll._ Lower
layer cells. _m._ Mesoblast. _n._ Nuclei of yolk of Selachian egg. _nc._
Neural canal. _sg._ Segmentation cavity. _x._ Point where epiblast and
hypoblast are continuous at the mouth of the alimentary involution. This
point is always situated at the tail end of the embryo. _yk._ Yolk.
Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower
layer cells before their separation into hypoblast and mesoblast are also
coloured green.
A I, A II, A III. Diagrammatic sections of Amphioxus in its early stages
(founded upon Kowalevsky's observations).
B I, B II, B III. Diagrammatic longitudinal sections of an hypothetical
animal, intermediate between Amphioxus and Batrachians, in its early
stages.
C I, C II, C III. Diagrammatic longitudinal sections of Bombinator igneus
in its early stages (founded upon Götte's observations). In C III the
neural canal is completed, which was not the case in B III. The epiblast in
C III has been diagrammatically represented as a single layer.
D I, D II, D III. Diagrammatic longitudinal sections of an animal,
intermediate between Batrachians and Selachians, in its early stages.
E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its
early stages.
E´. Surface view of the yolk of a Selachian's egg to shew the manner in
which it is enclosed by the Blastoderm. The yolk is represented yellow and
the Blastoderm blue.
F I, F II, F III. Diagrammatic longitudinal sections of a Bird in its early
stages.
VII. ON THE ORIGIN AND HISTORY OF THE URINOGENITAL ORGANS OF
VERTEBRATES[26].
Footnote 26: From the _Journal of Anatomy and Physiology_,
Vol. X. 1875.
Recent discoveries[27] as to the mode of development and anatomy of the
urinogenital system of Selachians, Amphibians, and Cyclostome fishes, have
greatly increased our knowledge of this system of organs, and have rendered
more possible a comparison of the types on which it is formed in the
various orders of vertebrates.
Footnote 27: The more important of these are:--
Semper--Ueber die Stammverwandtschaft der Wirbelthiere u.
Anneliden. _Centralblatt f. Med. Wiss._ 1874, No. 35.
Semper--Segmentalorgane bei ausgewachsenen Haien. _Centralblatt
f. Med. Wiss._ 1874, No. 52.
Semper--Das Urogenitalsystem der höheren Wirbelthiere.
_Centralblatt f. Med. Wiss._ 1874, No. 59.
Semper--Stammesverwandtschaft d. Wirbelthiere u. Wirbellosen.
_Arbeiten aus Zool. Zootom. Inst._ Würzburg. II Band.
Semper--Bildung u. Wachstum der Keimdrüsen bei den
Plagiostomen. _Centralblatt f. Med. Wiss._ 1875, No. 12.
Semper--Entw. d. Wolf. u. Müll. Gang. _Centralblatt f. Med.
Wiss._ 1875, No. 29.
Alex. Schultz--Phylogenie d. Wirbelthiere. _Centralblatt f.
Med. Wiss._ 1874, No. 51.
Spengel--Wimpertrichtern i. d. Amphibienniere. _Centralblatt f.
Med. Wiss._ 1875, No. 23.
Meyer--Anat. des Urogenitalsystems der Selachier u. Amphibien.
_Sitzb. Naturfor. Gesellschaft._ Leipzig, 30 April, 1875.
F. M. Balfour--Preliminary Account of development of
Elasmobranch fishes. _Quart. Journ. of Micro. Science_, Oct.
1874. (This edition, Paper V. p. 60 _et seq._)
W. Müller--Persistenz der Urniere bei Myxine glutinosa.
_Jenaische Zeitschrift_, 1873.
W. Müller--Urogenitalsystem d. Amphioxus u. d. Cyclostomen.
_Jenaische Zeitschrift_, 1875.
Alex. Götte--_Entwicklungsgeschichte der Unke (Bombinator
igneus)._
The following paper is an attempt to give a consecutive history of the
origin of this system of organs in vertebrates and of the changes which it
has undergone in the different orders.
For this purpose I have not made use of my own observations alone, but have
had recourse to all the Memoirs with which I am acquainted, and to which I
have access. I have commenced my account with the Selachians, both because
my own investigations have been directed almost entirely to them, and
because their urinogenital organs are, to my mind, the most convenient for
comparison both with the more complicated and with the simpler types.
On many points the views put forward in this paper will be found to differ
from those which I expressed in my paper (_loc. cit._) which give an
account of my original[28] discovery of the segmental organs of Selachians,
but the differences, with the exception of one important error as to the
origin of the Wolffian duct, are rather fresh developments of my previous
views from the consideration of fresh facts, than radical changes in them.
Footnote 28: These organs were discovered independently by
Professor Semper and myself. Professor Semper's preliminary
account appeared prior to my own which was published (with
illustrations) in the _Quarterly Journal of Mic. Science_.
Owing to my being in South America, I did not know of Professor
Semper's investigations till several months after the
publication of my paper.
* * * * *
In Selachian embryos an intermediate cell-mass, or middle plate of
mesoblast is formed, as in birds, from a partial fusion of the somatic and
splanchnic layers of the mesoblast at the outer border of the
protovertebræ. From this cell-mass the whole of the urinogenital system is
developed.
At about the time when three visceral clefts have appeared, there arises
from the intermediate cell-mass, opposite the fifth protovertebra, a solid
knob, from which a column of cells grows backwards to opposite the position
of the future anus (Fig. 1. _pd._).
[Illustration: FIG. 1. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE
VISCERAL CLEFTS.
The sections are to shew the development of the segmental duct (_pd_) or
primitive duct of the kidneys. In _A_ (the anterior of the two sections)
this appears as a solid knob projecting towards the epiblast. In _B_ is
seen a section of the column which has grown backwards from the knob in
_A_.
_spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._ notochord;
_X._ string of cells below the notochord; _mp._ muscle-plate; _mp´._
specially developed portion of muscle-plate; _ao._ dorsal aorta; _pd._
segmental duct; _so._ somatopleura; _sp._ splanchnopleura; _pp._
pleuro-peritoneal or body-cavity; _ep._ epiblast; _al._ alimentary canal.]
This knob projects outwards toward the epiblast, and the column lies at
first between the mesoblast and epiblast. The knob and column do not long
remain solid. The knob becoming hollow acquires a wide opening into the
pleuro-peritoneal or body-cavity, and the column a lumen; so that by the
time that five visceral clefts have appeared, the two together form a duct
closed behind, but communicating in front by a wide opening with the
pleuro-peritoneal cavity.
Before these changes are accomplished, a series of _solid_[29] outgrowths
of elements of the 'intermediate cell-mass' appear at the uppermost corner
of the body-cavity. These soon become hollow and appear as involutions from
the body-cavity, curling round the inner and dorsal side of the previously
formed duct.
Footnote 29: These outgrowths are at first solid in both
Pristiurus, Scyllium and Torpedo, but in Torpedo attain a
considerable length before a lumen appears in them.
One involution of this kind makes its appearance for each protovertebra,
and the first belongs to the protovertebra immediately behind the anterior
end of the duct whose development has just been described. In Pristiurus
there are in all 29 of these at this period. The last two or three arise
from that portion of the body-cavity, which at this stage still exists
behind the anus. The first-formed duct and the subsequent involutions are
the rudiments of the whole of the urinary system. The duct is the primitive
duct of the kidney[30]; I shall call it in future _the segmental duct_; and
the involutions are the commencements of the segmental tubes which
constitute the body of the kidney. I shall call them in future _segmental
tubes_.
Footnote 30: This duct is often called either Müller's duct,
the oviduct, or the duct of the primitive kidneys
'Urnierengang.' None of these terms are very suitable. A
justification of the name I have given it will appear from the
facts given in the later parts of this paper. In my previous
paper I have always called it oviduct, a name which is very
inappropriate.
Soon after their formation the segmental tubes become convoluted, and their
blind ends become connected with the segmental duct of the kidney. At the
same time, or rather before this, the blind posterior termination of each
of the segmental ducts of the kidneys unites with and opens into one of the
horns of the cloaca. At this period the condition of affairs is represented
in Fig. 2.
[Illustration: FIG. 2. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN
A SELACHIAN EMBRYO.
_pd._ segmental duct. It opens at _o_ into the body-cavity and at its other
extremity into the cloaca; _x._ line along which the division appears which
separates the segmental duct into the Wolffian duct above and the Müllerian
duct below; _st._ segmental tubes. They open at one end into the
body-cavity, and at the other into the segmental duct.]
There is at _pd_, the segmental duct of the kidneys, opening in front (_o_)
into the body-cavity, and behind into the cloaca, and there are a series of
convoluted segmental tubes (_st_), each opening at one end into the
body-cavity, and at the other into the duct (_pd_).
The next important change which occurs is the longitudinal division of the
segmental duct of the kidneys into Müller's duct, or the oviduct, and the
duct of the Wolffian bodies or Leydig's duct. The splitting[31] is effected
by the growth of a wall of cells which divides the duct into two parts
(fig. 3, _wd._ and _md._). It takes place in such a way that the front end
of the segmental duct, anterior to the entrance of the first segmental
tube, together with the ventral half of the rest of the duct, is split off
from its dorsal half as an independent duct (vide fig. 2, _x_).
Footnote 31: This splitting was first of all discovered and an
account of it published by Semper (_Centralblatt f. Med. Wiss._
1875, No. 29). I had independently made it out for the female a
few weeks before the publication of Semper's account--but have
not yet made observations about the point for the male.
My own previous account of the origin of the Wolffian duct
(_Quart. Journ. of Micros. Science_, Oct. 1874, and this
edition, Paper V.), is completely false, and was due to my not
having had access to a complete series of my sections when I
wrote the paper.
[Illustration: FIG. 3. TRANSVERSE SECTION OF A SELACHIAN EMBRYO
ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MÜLLERIAN DUCTS BY THE
LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.
_mc._ medullary canal; _mp._ muscle-plate; _ch._ notochord; _ao._ aorta;
_cav._ cardinal vein; _st._ segmental tube. On the one side the section
passes through the opening of a segmental tube into the body-cavity. On the
other this opening is represented by dotted lines, and the opening of the
segmental tube into the Wolffian duct has been cut through; _wd._ Wolffian
duct; _md._ Müllerian duct. The Müllerian duct and the Wolffian duct
together constitute the primitive segmental duct; _gr._ The germinal ridge
with the thickened germinal epithelium; _l._ liver; _i._ intestine with
spiral valve.]
The dorsal portion also forms an independent duct, and into it the
segmental tubes continue to open. Such at least is the method of splitting
for the female--for the male the splitting is according to Professor
Semper, of a more partial character, and consists for the most part in the
front end of the duct only being separated off from the rest. The result of
these changes is the formation in both sexes of a fresh duct which carries
off the excretions of the segmental involutions, and which I shall call the
Wolffian duct--while in the female there is formed another complete and
independent duct, which I shall call the Müllerian duct, or oviduct, and in
the male portions only of such a duct.
The next change which takes place is the formation of another duct from the
hinder portion of the Wolffian duct, which receives the secretion of the
posterior segmental tubes. This secondary duct unites with the primary or
Wolffian duct near its termination, and the primary ducts of the two sides
unite together to open to the exterior by a common papilla.
Slight modifications of the posterior terminations of these ducts are found
in different genera of Selachians (vide Semper, _Centralblatt für Med.
Wiss_. 1874, No. 59), but they are of no fundamental importance.
These constitute the main changes undergone by the segmental duct of the
kidneys and the ducts derived from it; but the segmental tubes also undergo
important changes. In the majority of Selachians their openings into the
body-cavity, or, at any rate, the openings of a large number of them,
persist through life; but the investigations of Dr Meyer[32] render it very
probable that the small portion of each segmental tube adjoining the
opening becomes separated from the rest and becomes converted into a sort
of lymph organ, so that the openings of the segmental tubes in the adult
merely lead into lymph organs and not into the gland of the kidneys.
Footnote 32: _Sitzun. der Naturfor. Gesellschaft_, Leipzig, 30
April, 1875.
These constitute the whole changes undergone in the female, but in the male
the open ends of a varying number (according to the species) of the
segmental tubes become connected with the testis and, uniting with the
testicular follicles, serve to carry away the seminal fluid[33]. The
spermatozoa have therefore to pass through a glandular portion of the
kidneys before they enter the Wolffian duct, by which they are finally
carried away to the exterior.
Footnote 33: We owe to Professor Semper the discovery of the
arrangement of the seminal ducts. _Centralblatt f. Med. Wiss._
1875, No. 12.
In the adult female, then, there are the following parts of the
urinogenital system (fig. 4):
(1) The oviduct, or Müller's duct (fig. 4, _md._), split off from the
segmental duct of the kidneys. Each oviduct opens at its upper end into the
body-cavity, and behind the two oviducts have independent communications
with the cloaca. The oviducts serve simply to carry to the exterior the
ova, and have no communication with the glandular portion of the kidneys.
[Illustration: FIG. 4. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL
ORGANS IN AN ADULT FEMALE SELACHIAN.
_md._ Müllerian duct; _wd._ Wolffian duct; _st._ segmental tubes; _d._ duct
of the posterior segmental tubes; _ov._ ovary.]
(2) The Wolffian ducts (fig. 4, _wd._) or the remainder of the segmental
ducts of the kidneys. Each Wolffian duct ends blindly in front, and the two
unite behind to open by a common papilla into the cloaca.
This duct receives the secretion of the whole anterior end of the
kidneys[34], that is to say, of all the anterior segmental tubes.
Footnote 34: This upper portion of the kidneys is called
Leydig's gland by Semper. It would be better to call it the
Wolffian body, for I shall attempt to shew that it is
homologous with the gland so named in Sauropsida and Mammalia.
(3) The secondary duct (fig. 4, _d._) belonging to the lower portion of the
kidneys opening into the former duct near its termination.
(4) The segmental tubes (fig. 4, _st_) from whose convolutions and
outgrowths the kidney is formed. They may be divided into two parts,
according to the duct by which their secretion is carried off.
In the male the following parts are present:
(1) The Müllerian duct (fig. 5, _md._), consisting of a small remnant,
attached to the liver, which represents the foremost end of the oviduct of
the female.
(2) The Wolffian duct (fig. 5, _wd_), which precisely corresponds to the
Wolffian duct of the female, except that, in addition to functioning as the
duct of the anterior part of the kidneys, it also serves to carry away the
semen. In the female it is straight, but has in the adult male a very
tortuous course (vide fig. 5).
[Illustration: FIG. 5. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL
ORGANS IN AN ADULT MALE SELACHIAN.
_md._ rudiment of Müllerian duct; _wd._ Wolffian duct, which also serves as
vas deferens; _st._ segmental tubes. The ends of three of those which in
the female open into the body-cavity, have in the male united with the
testicular follicles, and serve to carry away the products of the testis;
_d._ duct of the posterior segmental tubes; _t._ testis.]
(3) the duct (fig. 5, _d._) of the posterior portion of the kidneys, which
has the same relations as in the female.
(4) The segmental tubes (fig. 5, _st._). These have the same relations as
in the female, except that the most anterior two, three or more, unite with
the testicular follicles, and carry away the semen into the Wolffian duct.
* * * * *
The mode of arrangement and the development of these parts suggest a number
of considerations.
In the first place it is important to notice that the segmental tubes
develop primitively as completely independent organs[35], one of which
appears in each segment. If embryology is in any way a repetition of
ancestral history, it necessarily follows that these tubes were primitively
independent of each other. Ancestral history, as recorded in development,
is often, it is true, abridged; but it is clear that though abridgement
might prevent a series of primitively separate organs from appearing as
such, yet it would hardly be possible for a primitively compound organ,
which always retained this condition, to appear during development as a
series of separate ones. These considerations appear to me to prove that
the segmented ancestors of vertebrates possessed a series of independent
and segmental excretory organs.
Footnote 35: Further study of my sections has shewn me that
the initial independence of these organs is even more complete
than might be gathered from the description in my paper (_loc.
cit._). I now find, as I before conjectured, that they at first
correspond exactly with the muscle-plates, there being one for
each muscle-plate. This can be seen in the fresh embryos, but
longitudinal sections shew it in an absolutely demonstrable
manner.
Both Professor Semper and myself, on discovering these organs, were led to
compare them and state our belief in their identity with the so-called
segmental organs of Annelids.
This view has since been fairly generally accepted. The segmental organs of
annelids agree with those of vertebrates in opening at one end into the
body-cavity, but differ in the fact that each also communicates with the
exterior by an independent opening, and that they are never connected with
each other.
On the hypothesis of the identity of the vertebrate segmental tubes with
the annelid segmental organs, it becomes essential to explain how the
external openings of the former may have become lost.
This brings us at once to the origin of the segmental duct of the kidneys,
by which the secretion of all the segmental tubes was carried to the
exterior, and it appears to me that a right understanding of the vertebrate
urinogenital system depends greatly upon a correct view of the origin of
this duct. I would venture to repeat the suggestion which I made in my
original paper (_loc. cit._) that this duct is to be looked upon as the
most anterior of the segmental tubes which persist in vertebrates. In
favour of this view are the following anatomical and embryological facts.
(1) It develops in nearly the same manner as the other segmental tubes,
viz. in Selachians as a solid outgrowth from the intermediate cell-mass,
which subsequently becomes hollowed so as to open into the body-cavity: and
in Amphibians and Osseous and Cyclostome fishes as a direct involution from
the body-cavity. (2) In Amphibians, Cyclostomes and Osseous fishes its
upper end develops a glandular portion, by becoming convoluted in a manner
similar to the other segmental tubes. This glandular portion is often
called either the head-kidney or the primitive kidney. It is only an
embryonic structure, but is important as demonstrating the true nature of
the primitive duct of the kidneys.
We may suppose that some of the segmental tubes first united, possibly in
pairs, and that then by a continuation of this process the whole of them
coalesced into a common gland. One external opening sufficed to carry off
the entire secretion of the gland, and the other openings therefore
atrophied.
This history is represented in the development of the dog-fish in an
abbreviated form, by the elongation of the first segmental tube (segmental
duct of the kidney) and its junction with each of the posterior segmental
tubes. Professor Semper looks upon the primitive duct of the kidneys as a
duct which arose independently, and was not derived from metamorphosis of
the segmental organs. Against this view I would on the one hand urge the
consideration, that it is far easier to conceive of the transformation by
change of function (comp. Dohrn, _Functionswechsel_, Leipzig, 1875) of a
segmental organ into a segmental duct, than to understand the physiological
cause which should lead, in the presence of so many already formed ducts,
to the appearance of a totally new one. By its very nature a duct is a
structure which can hardly arise de novo. We must even suppose that the
segmental organs of Annelids were themselves transformations of still
simpler structures. On the other hand I would point to the development in
this very duct amongst Amphibians and Osseous fishes of a glandular portion
similar to that of a segmental tube, as an _à posteriori_ proof of its
being a metamorphosed segmental tube. The development in insects of a
longitudinal tracheal duct by the coalescence of a series of transverse
tracheal tubes affords a parallel to the formation of a duct from the
coalescence of a series of segmental tubes.
Though it must be admitted that the loss of the external openings of the
segmental organs requires further working out, yet the difficulties
involved in their disappearance are not so great as to render it improbable
that the vertebrate segmental organs are descended from typical annelidan
ones.
The primitive vertebrate condition, then, is probably that of an early
stage of Selachian development while there is as yet a segmental duct,--the
original foremost segmental tube opening in front into the body-cavity and
behind into the cloaca; with which duct all the segmental tubes
communicate. Vide Fig. 2.
The next condition is to be looked upon as an indirect result of the
segmental duct serving as well for the products of the generative organs as
the secretions of the segmental tubes.
As a consequence of this, the segmental duct became split into a ventral
portion, which served alone for the ova, and a dorsal portion which
received the secretion of the segmental tubes. The lower portion, which we
have called the oviduct, in some cases may also have received the semen as
well as the ova. This is very possibly the case with Ceratodus (vide
Günther, _Trans. of Royal Society_, 1871), and the majority of Ganoids
(Hyrtl, _Denkschriften Wien_, Vol. VIII.). In the majority of other cases
the oviduct exists in the male in a completely rudimentary form; and the
semen is carried away by the same duct as the urine.
In Selachians the transportation of the semen from the testis to the
Wolffian duct is effected by the junction of the open ends of two or three
or more segmental tubes with the testicular follicles, and the modes in
which this junction is effected in the higher vertebrates seem to be
derivatives from this. If the views here expressed are correct it is by a
complete change of function that the oviduct has come to perform its
present office. And in the bird and higher vertebrates no trace, or only
the very slightest (vide p. 165) of the primitive urinary function is
retained during embryonic or adult life.
The last feature in the anatomy of the Selachians which requires notice is
the division of the kidney into two portions, an anterior and posterior.
The anatomical similarity between this arrangement and that of higher
vertebrates (birds, &c.) is very striking. The anterior one precisely
corresponds, anatomically, to the _Wolffian body_, and the posterior one to
the true permanent _kidney_ of higher vertebrates: and when we find that in
the Selachians the duct for the anterior serves also for the semen as does
the Wolffian duct of higher vertebrates, this similarity seems almost to
amount to identity. A discussion of the differences in development in the
two cases will come conveniently with the account of the bird; but there
appear to me the strongest grounds for looking upon the kidneys of
Selachians as equivalent to both the Wolffian bodies and the true kidneys
of the higher vertebrates.
The condition of the urinogenital organs in Selachians is by no means the
most primitive found amongst vertebrates.
The organs of both Cyclostomous and Osseous fishes, as well as those of
Ganoids, are all more primitive; and in the majority of points the
Amphibians exhibit a decidedly less differentiated condition of these
organs than do the Selachians.
In Cyclostomous fishes the condition of the urinary system is very simple.
In Myxine (vide Joh. Müller _Myxinoid fishes_, and Wilhelm Müller,
_Jenaische Zeitschrift_, 1875, _Das Urogenitalsystem des Amphioxus u. d.
Cyclostomen_) there is a pair of ducts which communicate posteriorly by a
common opening with the abdominal pore. From these ducts spring a series of
transverse tubules, each terminating in a Malpighian corpuscle. These
together constitute the mass of the kidneys. About opposite the
gall-bladder the duct of the kidney (the segmental duct) narrows very much,
and after a short course ends in a largish glandular mass (the
head-kidney), which communicates with the pericardial cavity by a number of
openings.
In Petromyzon the anatomy of the kidneys is fundamentally the same as in
Myxine. They consist of the two segmental ducts, and a number of fine
branches passing off from these, which become convoluted but do not form
Malpighian tufts. The head-kidney is absent in the adult.
W. Müller (_loc. cit._) has given a short but interesting account of the
development of the urinary system of Petromyzon. He finds that the
segmental ducts develop first of all as simple involutions from the
body-cavity. The anterior end of each then develops a glandular portion
which comes to communicate by a number of openings with the body-cavity.
Subsequently to the development of this glandular portion the remainder of
the kidneys appears in the posterior portion of the body-cavity; and before
the close of embryonic life the anterior glandular portion atrophies.
The comparison of this system with that of a Selachian is very simple. The
first developed duct is the segmental duct of a Selachian, and the
glandular portion developed at its anterior extremity, which is permanent
in Myxine but embryonic in Petromyzon, is, as W. Müller has rightly
recognized, equivalent to the head-kidney of Amphibians, which remains
undeveloped in Selachians. It is, according to my previously stated view,
the glandular portion of the first segmental organ or the segmental duct.
The series of orifices by which this communicates with the body-cavity are
due to the division of the primary opening of the segmental duct. This is
shewn both by the facts of their development in Petromyzon given by Müller,
as well as by the occurrence of a similar division of the primary orifice
in Amphibians, which is mentioned later in this paper. In a note in my
original paper (_loc. cit._) I stated that these openings were equivalent
to the segmental involutions of Selachians. This is erroneous, and was due
to my not having understood the description given in a preliminary paper of
Müller (_Jenaische Zeitschrift_, 1873). The large development of this
glandular mass in the Cyclostome and Osseous fishes and in embryo
Amphibians, implies that it must at one time have been important. Its
earlier development than the remainder of the kidneys is probably a result
of the specialized function of the first segmental organ.
The remainder of the kidney in Cyclostomes is equivalent to the kidney of
Selachians. Its development from segmental involutions has not been
recognized. If these segmental involutions are really absent it may perhaps
imply that the simplicity of the Cyclostome kidneys, like that of so many
other of their organs, is a result of degeneration rather than a primitive
condition.
In Osseous fishes the segmental duct of the kidneys develops, as the
observations of Rosenberg[36] ("Teleostierniere," _Inaug. Disser. Dorpat_,
1867) and Oellacher (_Zeitschrift für Wiss. Zool._ 1873) clearly prove, by
an involution from the body-cavity. This involution grows backwards in the
form of a duct and opens into the cloaca. The upper end of this duct (the
most anterior segmental tube) becomes convoluted, and forms a glandular
body, which has no representative in the urinary apparatus of Selachians,
but whose importance, as indicating the origin of the segmental duct of the
kidneys, I have already insisted upon.
Footnote 36: I am unfortunately only acquainted with Dr
Rosenberg's paper from an abstract.
The rest of the kidney becomes developed at a later period, probably in the
same way as in Selachians; but this, as far as I know, has not been made
out.
The segmental duct of the kidneys forms the duct for this new gland, as in
embryo Selachians (Fig. 2), but, unlike what happens in Selachians,
undergoes no further changes, with the exception of a varying amount of
retrogressive metamorphosis of its anterior end. The kidneys of Osseous
fish usually extend from just behind the head to opposite the anus, or even
further back than this. They consist for the most part of a broader
anterior portion, an abdominal portion reaching from this to the anus, and,
as in those cases in which the kidneys extend further back than the anus,
of a caudal portion.
The two ducts (segmental ducts of the kidneys) lie, as a rule, in the lower
part of the kidneys on their outer borders, and open almost invariably into
a urinary bladder. In some cases they unite before opening into the
bladder, but generally have independent openings.
This bladder, which is simply a dilatation of the united lower ends of the
primitive kidney-ducts, and has no further importance, is almost invariably
present, but in many cases lies unsymmetrically either to the right or the
left. It opens to the exterior by a very minute opening in the
genito-urinary papilla, immediately behind the genital pore. There are,
however, a few cases in which the generative and urinary organs have a
common opening. For further details vide Hyrtl, _Denk. der k. Akad. Wien_,
Vol. II.
It is possible that the generative ducts of Osseous fishes are derived from
a splitting from the primitive duct of the kidney, but this is discussed
later in the paper.
In Osseous fishes we probably have an embryonic condition of the Selachian
kidneys retained permanently through life.
* * * * *
In the majority of Ganoids the division of the segmental duct of the kidney
into two would seem to occur, and the ventral duct of the two (Müllerian
duct), which opens at its upper end into the body-cavity, is said to serve
as an excretory duct for both male and female organs.
The following are the more important facts which are known about the
generative and urinary ducts of Ganoids.
In Spatularia (vide Hyrtl, Geschlechts u. Harnwerkzeuge bei den Ganoiden,
_Denkschriften der k. Akad. Wien_, Vol. VIII.) the following parts are
found in the female.
(1) The ovaries stretching along the whole length of the abdominal cavity.
(2) The kidneys, which are separate and also extend along the greater part
of the abdominal cavity.
(3) The ureters lying on the outer borders of the kidneys. Each ureter
dilates at its lower end into an elongated wide tube, which continues to
receive the ducts from the kidneys. The two ureters unite before
terminating and open behind the anus.
(4) The two oviducts (Müllerian ducts). These open widely into the
abdominal cavity, at about two-thirds of the distance from the anterior
extremity of the body-cavity. Each opens by a narrow pore into the dilated
ureter of its side.
In the male the same parts are found as in the female, but Hyrtl found that
the Müllerian duct of the left side at its entrance into the ureter became
split into two horns, one of which ended blindly. On the right side the
opening of the Müllerian duct was normal.
In the Sturgeon (vide J. Müller, _Bau u. Grenzen d. Ganoiden_, Berlin Akad.
1844; Leydig, _Fischen u. Reptilien_, and Hyrtl, _Ganoiden_) the same parts
are found as in Spatularia.
The kidneys extend along the whole length of the body-cavity; and the
ureter, which does not reach the whole length of the kidneys, is a
thin-walled wide duct lying on the outer side. On laying it open the
numerous apertures of the tubules for the kidney are exposed. The Müllerian
duct, which opens in both sexes into the abdominal cavity, ends, according
to Leydig, in the cases of some males, blindly behind without opening into
the ureter, and Müller makes the same statement for both sexes. It was open
on both sides in a female specimen I examined[37], and Hyrtl found it
invariably so in both sexes in all the specimens he examined.
Footnote 37: For this specimen I am indebted to Dr Günther.
Both Rathke and Stannius (I have been unable to refer to the original
papers) believed that the semen was carried off by transverse ducts
directly into the ureter, and most other observers have left undecided the
mechanism of the transportation of the semen to the exterior. If we suppose
that the ducts Rathke saw really exist they might perhaps be supposed to
enter not directly into the ureter, but into the kidney, and be in fact
homologous with the vasa efferentia of the Selachians. The frequent blind
posterior termination of the Müllerian duct is in favour of the view that
these ducts of Rathke are really present.
In Polypterus (vide Hyrtl, _Ganoiden_) there is, as in other Ganoids, a
pair of Müllerian ducts. They unite at their lower ends. The ureters are
also much narrower than in previously described Ganoids and, after
coalescing, open into the united oviducts. The urinogenital canal, formed
by coalescence of the Müllerian ducts and ureters, has an opening to the
exterior immediately behind the anus.
In Amia (vide Hyrtl) there is a pair of Müllerian ducts which, as well as
the ureters, open into a dilated vesicle. This vesicle appears as a
continuation of the Müllerian ducts, but receives a number of the efferent
ductules of the kidneys. There is a single genito-urinary pore behind the
anus.
In Ceratodus (Günther, _Phil. Trans._ 1871) the kidneys are small and
confined to the posterior extremity of the abdomen. The generative organs
extend however along the greater part of the length of the abdominal
cavity. In both male and female there is a long Müllerian duct, and the
ducts of the two sides unite and open by a common pore into a urinogenital
cloaca which communicates with the exterior by the same opening as the
alimentary canal. In both sexes the Müllerian duct has a wide opening near
the anterior extremity of the body-cavity. The ureters coalesce and open
together into the urinogenital cloaca dorsal to the Müllerian ducts. It is
not absolutely certain that the semen is transported to the exterior by the
Müllerian duct of the male, which is perhaps merely a rudiment as in
Amphibia. Dr Günther failed however to find any other means by which it
could be carried away.
The genital ducts of Lepidosteus differ in important particulars from those
of the other Ganoids (vide Müller, _loc. cit._ and Hyrtl, _loc. cit._).
In both sexes the genital ducts are continuous with the investments of the
genital organs.
In the female the dilated posterior extremities of the ureters completely
invest for some distance the generative ducts, whose extremities are
divided into several processes, and end in a different way on the two
sides. A similar division and asymmetry of the ducts is mentioned by Hyrtl
as occurring in the male of Spatularia, and it seems not impossible that on
the hypothesis of the genital ducts being segmental tubes these divisions
may be remnants of primitive glandular convolutions. The ureters in both
sexes dilate as in other Ganoids at their posterior extremities, and unite
with one another. The unpaired urinogenital opening is situated behind the
anus. In the male the dilated portion of the ureters is divided into a
series of partitions which are not present in the female.
Till the embryology of the secretory system of Ganoids has been worked out,
the homologies of their generative ducts are necessarily a matter of
conjecture. It is even possible that what I have called the Müllerian duct
in the male is functionless, as with Amphibians, but that, owing to the
true ducts of the testis having been overlooked, it has been supposed to
function as the vas deferens. Günther's (_loc. cit._) injection experiments
on Ceratodus militate against this view, but I do not think they can be
considered as conclusive as long as the mechanism for the transportation of
the semen to the exterior has not been completely made out. Analogy would
certainly lead us to expect the ureter to serve in Ganoids as the vas
deferens.
The position of the generative ducts might in some cases lead to the
supposition that they are not Müllerian ducts, or, in other words, the most
anterior pair of segmental organs but a pair of the posterior segmental
tubes.
What are the true homologies of the generative ducts of Lepidosteus, which
are continuous with the generative glands, is somewhat doubtful. It is very
probable that they may represent the similarly functioning ducts of other
Ganoids, but that they have undergone further changes as to their anterior
extremities.
It is, on the other hand, possible that their generative ducts are the same
structures as those ducts of Osseous fishes, which are continuous with the
generative organs. These latter ducts are perhaps related to the abdominal
pores, and had best be considered in connection with these; but a
completely satisfactory answer to the questions which arise in reference to
them can only be given by a study of their development.
In the Cyclostomes the generative products pass out by an abdominal pore,
which communicates with the peritoneal cavity by two short tubes[38], and
which also receives the ducts of the kidneys.
Footnote 38: According to Müller (_Myxinoiden_, 1845) there is
in Myxine an abdominal pore with two short canals leading into
it, and Vogt and Pappenheim (_An. Sci. Nat._ Part IV. Vol. XI.)
state that in Petromyzon there are two such pores, each
connected with a short canal.
Gegenbaur suggests that these are to be looked upon as Müllerian ducts, and
as therefore developed from the segmental ducts of the kidneys. Another
possible view is that they are the primitive external openings of a pair of
segmental organs. In Selachians there are usually stated to be a pair of
abdominal pores. In Scyllium I have only been able to find, on each side, a
large deep pocket opening to the exterior, but closed below towards the
peritoneal cavity, so that in it there seem to be no abdominal pores[39].
In the Greenland Shark (_Læmargus Borealis_) Professor Turner (_Journal of
Anat. and Phys._ Vol. VIII.) failed to find either oviduct or vas deferens,
but found a pair of large open abdominal pores, which he believes serve to
carry away the generative products of both sexes. Whether the so-called
abdominal pores of Selachians usually end blindly as in Scyllium, or, as is
commonly stated, open into the body-cavity, there can be no question that
they are homologous with true abdominal powers.
Footnote 39: My own rough examination of preserved specimens
was hardly sufficient to enable me to determine for certain the
presence or absence of these pores. Mr Bridge, of Trinity
College, has, however, since then commenced a series of
investigations on this point, and informs me that these pores
are certainly absent in Scyllium as well as in other genera.
The blind pockets of Scyllium appear very much like the remains of
primitive involutions from the exterior, which might easily be supposed to
have formed the external opening of a pair of segmental organs, and this is
probably the true meaning of abdominal pores. The presence of abdominal
pores in all Ganoids in addition to true genital ducts and of these pockets
or abdominal pores in Selachians, which are almost certainly homologous
with the abdominal pores of Ganoids and Cyclostomes, and also occur in
addition to true Müllerian ducts, speak strongly against the view that the
abdominal pores have any relation to Müllerian ducts. Probably therefore
the abdominal pores of the Cyclostomous fishes (which seem to be of the
same character as other abdominal pores) are not to be looked on as
rudimentary Müllerian ducts.
We next come to the question which I reserved while speaking of the kidneys
of Osseous fishes, as to the meaning of their genital ducts.
In the female Salmon and the male and female Eel, the generative products
are carried to the exterior by abdominal pores, and there are no true
generative ducts. In the case of most other Osseous fish there are true
generative ducts which are continuous with the investment of the generative
organs[40] and have generally, though not always, an opening or openings
independent of the ureter close behind the rectum, but no abdominal pores
are present. It seems, therefore, that in Osseous fish the generative ducts
are complementary to abdominal pores, which might lead to the view that the
generative ducts were formed by a coalescence of the investment of the
generative glands with the short duct of abdominal pore.
Footnote 40: The description of the attachment of the vas
deferens to the testis in the Carp given by Vogt and Pappenheim
(_Ann. Scien. Nat._ 1859) does not agree with what I found in
the Perch (_Perca fluvialis_). The walls of the duct are in the
Perch continuous with the investment of the testis, and the
gland of the testis occupies, as it were, the greater part of
the duct; there is, however, a distinct cavity corresponding to
what Vogt and P. call the duct, near the border of attachment
of the testis into which the seminal tubules open. I could find
at the posterior end of the testis no central cavity which
could be distinguished from the cavity of this duct.
Against this view there are, however, the following facts:
(1) In the cases of the salmon and the eel it is perfectly true that the
abdominal pore exactly corresponds with the opening of the genital duct in
other Osseous fishes, but the absence of genital ducts in these cases must
rather be viewed, as Vogt and Pappenheim (_loc. cit._) have already
insisted, as a case of degeneration than of a primitive condition. The
presence of genital ducts in the near allies of the Salmonidæ, and even in
the male salmon, are conclusive proofs of this. If we admit that the
presence of an abdominal pore in Salmonidæ is merely a result of
degeneration, it obviously cannot be used as an argument for the
complementary nature of abdominal pores and generative ducts.
(2) Hyrtl (_Denkschriften der k. Akad. Wien_, Vol. 1) states that in
Mormyrus oxyrynchus there is a pair of abdominal pores in addition to true
generative ducts. If his statements are correct, we have a strong argument
against the generative ducts of Osseous fishes being related to abdominal
pores. For though this is the solitary instance of the presence of both a
genital opening and abdominal pores known to me in Osseous fishes, yet we
have no right to assume that the abdominal pores of Mormyrus are not
equivalent to those of Ganoids and Selachians. It must be admitted, with
Gegenbaur, that embryology alone can elucidate the meaning of the genital
ducts of Osseous fishes.
In Lepidosteus, as was before mentioned, the generative ducts, though
continuous with the investment of the generative bodies, unite with the
ureters, and in this differ from the generative ducts of Osseous fishes.
The relation, indeed, of the generative ducts of Lepidosteus to the urinary
ducts is very similar to that existing in other Ganoid fishes; and this,
coupled with the fact that Lepidosteus possesses a pair of abdominal pores
on each side of the anus[41], makes it most probable that its generative
ducts are true Müllerian ducts.
Footnote 41: This is mentioned by Müller (_Ganoid fishes_,
Berlin Akad. 1844), Hyrtl (_loc. cit._), and Günther (_loc.
cit._), and through the courtesy of Dr Günther I have had an
opportunity of confirming the fact of the presence of the
abdominal pores on two specimens of Lepidosteus in the British
Museum.
* * * * *
In the Amphibians the urinary system is again more primitive than in the
Selachians.
The segmental duct of the kidneys is formed[42] by an elongated fold
arising from the outer wall of the body-cavity, in the same position as in
Selachians. This fold becomes constricted into a canal, closed except at
its anterior end, which remains open to the body-cavity. This anterior end
dilates, and grows out into two horns, and at the same time its opening
into the body-cavity becomes partly constricted, and so divided into three
separate orifices, one for each horn and a central one between the two. The
horns become convoluted, blood channels appearing between their
convolutions, and a special coil of vessels is formed arising from the
aorta and projecting into the body-cavity near the openings of the
convolutions. These formations together constitute the glandular
portion[43] of the original anterior segmental tube or segmental duct of
the kidneys. I have already pointed out the similarity which this organ
exhibits to the head-kidneys of Cyclostome fishes in its mode of formation,
especially with reference to the division of the primitive opening. The
lower end of the segmental duct unites with a horn of the cloaca.
Footnote 42: My account of the _development_ of these parts in
Amphibians is derived for the most part from Götte, _Die
Entwicklungsgeschichte der Unke_.
Footnote 43: It is called Kopfniere (head-kidney), or Urniere
(primitive kidney), by German authors. Leydig correctly looks
upon it as together with the permanent kidney constituting the
Urniere of Amphibians. The term Urniere is one which has arisen
in my opinion from a misconception; but certainly the Kopfniere
has no greater right to the appellation than the remainder of
the kidney.
After the formation of the gland just described the remainder of the kidney
is formed.
This arises in the same way as in Selachians. A series of involutions from
the body-cavity are developed; these soon form convoluted tubes, which
become branched and interlaced with one another, and also unite with the
primitive duct of the kidneys. Owing to the branching and interlacing of
the primitive segmental tubes, the kidney is not divided into distinct
segments in the same way as with the Selachians. The mode of development of
these segmental tubes was discovered by Götte. Their openings are ciliated,
and, as Spengel (_loc. cit._) and Meyer (_loc. cit._) have independently
discovered, persist in most adult Amphibians. As both these investigators
have pointed out, the segmental openings are in the adult kidneys of most
Amphibians far more numerous than the vertebral segments to which they
appertain. This is due to secondary changes, and is not to be looked upon
as the primitive state of things. At this stage the Amphibian kidneys are
nearly in the same condition as the Selachian, in the stage represented in
Fig. 2. In both there is the segmental duct of the kidneys, which is open
in front, communicates with the cloaca behind, and receives the whole
secretion from the kidneys. The parallelism between the two is closely
adhered to in the subsequent modifications of the Amphibian kidney, but the
changes are not completed so far in Amphibians as in Selachians. The
segmental duct of the Amphibian kidney becomes, as in Selachians, split
into a Müllerian duct or oviduct, and a Wolffian duct or duct for the
kidney.
The following points about this are noteworthy:
(1) The separation of the two ducts is never completed, so that they are
united together behind, and for a short distance, blend and form a common
duct; the ducts of the two sides so formed also unite before opening to the
exterior.
(2) The separation of the two ducts does not occur in the form of a simple
splitting, as in Selachians. But the efferent ductules from the kidney
gradually alter their points of entrance into the primitive duct. Their
points of entrance become carried backwards further and further, and since
this process affects the anterior ducts proportionally more than the
posterior, the efferent ducts finally all meet and form a common duct which
unites with the Müllerian duct near its posterior extremity. This process
is not always carried out with equal completeness. In the tailless
Amphibians, however, the process is generally[44] completed, and the
ureters (Wolffian ducts) are of considerable length. Bufo cinereus, in the
male of which the Müllerian ducts are very conspicuous, serves as an
excellent example of this.
Footnote 44: In Bombinator igneus, Von Wittich stated that the
embryonic condition was retained. Leydig, _Anatom. d. Amphib.
u. Reptilien_, shewed that this is not the case, but that in
the male the Müllerian duct is very small, though distinct.
In the Salamander (Salamandra maculosa), Figs. 6 and 7, the process is
carried out with greater completeness in the female than in the male, and
this is the general rule in Amphibians. In the male Proteus, the embryonic
condition would seem to be retained almost in its completeness so that the
ducts of the kidney open directly and separately into the still persisting
primitive duct of the kidney. The upper end of the duct nevertheless
extends some distance beyond the end of the kidney and opens into the
abdominal cavity. In the female Proteus, on the other hand, the separation
into a Müllerian duct and a ureter is quite complete. The Newt (Triton)
also serves as an excellent example of the formation of distinct Müllerian
and Wolffian ducts being much more complete in the female than the male. In
the female Newt all the tubules from the kidney open into a duct of some
length which unites with the Müllerian duct near its termination, but in
the male the anterior segmental tubes, including those which, as will be
afterwards seen, serve as vasa efferentia of the testis, enter the
Müllerian duct directly, while the posterior unite as in the female into a
common duct before joining the Müllerian duct. For further details as to
the variations exhibited in the Amphibians, the reader is referred to
Leydig, _Anat. Untersuchung, Fischen u. Reptilien_. Ditto, _Lehrbuch der
Histologie, Menschen u. Thiere_. Von Wittich, _Siebold u. Kölliker,
Zeitschrift_, Vol. IV. p. 125.
The different conditions of completeness of the Wolffian ducts observable
amongst the Amphibians are instructive in reference to the manner of
development of the Wolffian duct in Selachians. The _mode_ of division in
the Selachians of the segmental duct of the kidney into a Müllerian and
Wolffian duct is probably to be looked upon as an embryonic abbreviation of
the process by which these two ducts are formed in Amphibians. The fact
that this separation into Müllerian and Wolffian ducts proceeds further in
the females of most Amphibians than in the males, strikingly shews that it
is the oviductal function of the Müllerian duct which is the indirect cause
of its separation from the Wolffian duct. The Müllerian duct formed in the
way described persists almost invariably in both sexes, and in the male
sometimes functions as a sperm reservoir; _e.g._ Bufo cinereus. In the
embryo it carries at its upper end the glandular mass described above
(Kopfniere), but this generally atrophies, though remnants of it persist in
the males of some species (_e.g._ Salamandra). Its anterior end opens, in
most cases by a single opening, into the perivisceral cavity in both sexes,
and is usually ciliated. As the female reaches maturity, the oviduct
dilates very much; but it remains thin and inconspicuous in the male.
The only other developmental change of importance is the connection of the
testes with the kidneys. This probably occurs in the same manner as in
Selachians, viz. from the junction of the open ends of the segmental tubes
with the follicles of the testes. In any case the vessels which carry off
the semen constitute part of the kidney, and the efferent duct of the
testis is also that of the kidney. The vasa efferentia from the testis
either pass through one or two nearly isolated anterior portions of the
kidney (Proteus, Triton) or else no such special portion of the kidney
becomes separated from the rest, and the vasa efferentia enter the general
body of the kidney.
* * * * *
In the male Amphibian, then, the urinogenital system consists of the
following parts (Fig. 6):
(1) Rudimentary Müllerian ducts, opening anteriorly into the body-cavity,
which sometimes carry aborted _Kopfnieren_.
(2) The partially or completely formed Wolffian ducts (ureters) which also
serve as the ducts for the testes.
(3) The kidneys, parts of which also serve as the vasa efferentia, and
whose secretion, together with the testicular products, is carried off by
the Wolffian ducts.
(4) The united lower parts of Wolffian and Müllerian ducts which are really
the lower unsplit part of the segmental ducts of the kidneys.
[Illustration: FIG. 6. DIAGRAM OF THE URINOGENITAL ORGANS OF A MALE
SALAMANDER.
(_Copied from Leydig's Histologie des Menschen u. der Thiere._)
_md._ Müller's duct (rudimentary); _y._ remnant of the secretory portion of
the segmental duct Kopfniere; _Wd._ Wolffian duct; a less complete
structure in the male than in the female; _st._ segmental tubes or kidney.
The openings of these into the body-cavity are not inserted in the figure;
_t._ testis. Its efferent ducts form part of the kidney.]
In the female, there are (Fig. 7)
(1) The Müllerian ducts which function as the oviducts.
(2) The Wolffian ducts.
(3) The kidneys.
(4) The united Müllerian and Wolffian ducts as in the male.
[Illustration: FIG. 7. DIAGRAM OF THE URINOGENITAL ORGANS OF A FEMALE
SALAMANDER.
(_Copied from Leydig's Histologie des Menschen u. der Thiere_.)
_Md._ Müller's duct or oviduct; _Wd._ Wolffian duct or the duct of the
kidneys; _st._ segmental tubes or kidney. The openings of these into the
body-cavity are not inserted in the figure; _o._ ovary.]
The urinogenital organs of the adult Amphibians agree in almost all
essential particulars with those of Selachians. The ova are carried off in
both by a specialized oviduct. The Wolffian duct, or ureter, is found both
in Selachians and Amphibians, and the relations of the testis to it are the
same in both, the vasa efferentia of the testes having in both the same
anatomical peculiarities.
The following points are the main ones in which Selachians and Amphibians
differ as to the anatomy of the urinogenital organs; and in all but one of
these, the organs of the Amphibian exhibit a less differentiated condition
than do those of the Selachian.
(1) A glandular portion (Kopfniere) belonging to the first segmental organ
(segmental duct of the kidneys) is found in all embryo Amphibians, but
usually disappears, or only leaves a remnant in the adult. It has not yet
been found in any Selachian.
(2) The division of the primitive duct of the kidney into the Müllerian
duct and the Wolffian duct is not completed so far in Amphibians as
Selachians, and in the former the two ducts are confluent at their lower
ends.
(3) The permanent kidney exhibits in Amphibians no distinction into two
glands (foreshadowing the Wolffian bodies and true kidneys of higher
vertebrates), as it does in the Selachians.
(4) The Müllerian duct persists in its entirety in male Amphibians, but
only its upper end remains in male Selachians.
(5) The openings of the segmental tubes into the body-cavity correspond in
number with the vertebral segments in most Selachians, but are far more
numerous than these in Amphibians. This is the chief point in which the
Amphibian kidney is more differentiated than the Selachian.
* * * * *
The modifications in development which the urinogenital system has suffered
in higher vertebrates (Sauropsida and Mammalia) are very considerable;
nevertheless it appears to me to be possible with fair certainty to trace
out the relationship of its various parts in them to those found in the
Ichthyopsida. The development of urinogenital organs has been far more
fully worked out for the bird than for any other member of the amniotic
vertebrates; but, as far as we know, there are no essential variations
except in the later periods of development throughout the division. These
later variations, concerning for the most part the external apertures of
the various ducts, are so well known and have been so fully described as to
require no notice here. The development of these parts in the bird will
therefore serve as the most convenient basis for comparison.
In the bird the development of these parts begins by the appearance of a
column of cells on the upper surface of the intermediate cell-mass (Fig. 8,
_W.d_). As in Selachians, the intermediate cell-mass is a group of cells
between the outer edge of the protovertebræ and the upper end of the
body-cavity. The column of cells thus formed is the commencement of the
duct of the Wolffian body. Its development is strikingly similar to that of
the segmental duct of the kidney in Selachians. I shall attempt when I have
given an account of the development of the Müllerian duct to speak of the
relations between the Selachian duct and that of the bird.
Romiti (_Archiv f. Micr. Anat._ Vol. X.) has recently stated that the
Wolffian duct develops as an involution from the body-cavity. The fact that
the specimens drawn by Romiti to support this view are too old to determine
such a point, and the inspection of a number of specimens made by my friend
Mr Adam Sedgwick of Trinity College, who, at my request, has been examining
the urinogenital organs of the fowl, have led me to the conclusion that
Romiti is in error in differing from his predecessors as to the development
of the Wolffian duct. The solid string of cells to form the Wolffian duct
lies at first close to the epiblast, but, by the alteration in shape which
the protovertebræ undergo and the general growth of cells around it,
becomes gradually carried downwards till it lies close to the germinal
epithelium which lines the body-cavity. While undergoing this change of
position it also acquires a lumen, but ends blindly both in front and
behind. Towards the end of the fourth day the Wolffian duct opens into a
horn of the cloaca. The cells adjoining its inner border commence, as it
passes down on the third day, to undergo histological changes, which, by
the fourth day, result in the formation of a series of ducts and Malpighian
tufts which form the mass of the Wolffian body[45].
Footnote 45: This account of the origin of the Wolffian body
differs from that given by Waldeyer, and by Dr Foster and
myself (_Elements of Embryology_, Foster and Balfour), but I
have been led to alter my view from an inspection of Mr
Sedgwick's preparations, and I hope to shew that theoretical
considerations lead to the expectation that the Wolffian body
would develop independently of the duct.
[Illustration: FIG. 8. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN
EMBRYO FOWL OF 45 H. TO SHEW THE MODE OF FORMATION OF THE WOLFFIAN DUCT.
_A._ epiblast; _B._ mesoblast; _C._ hypoblast; _M.c._ medullary canal;
_Pv._ Protovertebræ; _W.d._ Wolffian duct; _So._ Somatopleure; _Sp._
Splanchnopleure; _pp._ pleuro-peritoneal cavity; _ch._ notochord; _ao._
dorsal aorta; _v._ blood-vessels.]
The Müllerian duct arises in the form of an involution, whether at first
solid or hollow, of the germinal epithelium, and, as I am satisfied, quite
independently of the Wolffian duct. It is important to notice that its
posterior end soon unites with the Wolffian duct, from which however it not
long after becomes separated and opens independently into the cloaca. The
upper end remains permanently open to the body-cavity, and is situated
nearly opposite the extreme front end of the Wolffian body.
Between the 80th and 100th hour of incubation the ducts of the permanent
kidneys begin to make their appearance. Near its posterior extremity each
Wolffian duct becomes expanded, and from the dorsal side of this portion a
diverticulum is constricted off, the blind end of which points forwards.
This is the duct of the permanent kidneys, and around its end the kidneys
are found. It is usually stated that the tubules of the permanent kidneys
arise as outgrowths from the duct, but this requires to be worked over
again.
The condition of the urinogenital system in birds immediately after the
formation of the permanent kidneys is strikingly similar to its permanent
condition in adult Selachians. There is the Müllerian duct in both opening
in front into the body-cavity and behind into the cloaca. In both the
kidneys consist of two parts--an anterior and posterior--which have been
called respectively Wolffian bodies and permanent kidneys in birds and
Leydig's glands and the kidneys in Selachians.
The duct of the permanent kidney, which at first opens into that of the
Wolffian body, subsequently becomes further split off from the Wolffian
duct, and opens independently into the cloaca.
The subsequent changes of these parts are different in the two sexes.
In the female the Müllerian ducts[46] persist and become the oviducts.
Their anterior ends remain open to the body-cavity. The changes in their
lower ends in the various orders of Sauropsida and Mammalia are too well
known to require repetition here. The Wolffian body and duct atrophy: there
are left however in many cases slight remnants of the anterior extremity of
the body forming the parovarium of the bird, and also frequently remnants
of the posterior portion of the gland as well as of the duct. The permanent
kidney and its duct remain unaltered.
Footnote 46: The right oviduct atrophies in birds, and the
left alone persists in the adult.
In the male the Müllerian duct becomes almost completely obliterated. The
Wolffian duct persists and forms the vas deferens, and the anterior
so-called sexual portion of the Wolffian body also persists in an altered
form. Its tubules unite with the seminiferous tubules, and also form the
epididymis. Unimportant remnants of the posterior part of the Wolffian body
also persist, but are without function. In both sexes the so-called
permanent kidneys form the sole portion of the primitive uriniferous system
which persists in the adult.
In considering the relations between the modes of development of the
urinogenital organs of the bird and of the Selachians, the first important
point to notice is, that whereas in the Selachians the segmental duct of
the kidneys is first developed and subsequently becomes split into the
Müllerian and Wolffian ducts; in the bird these two ducts develop
independently. This difference in development would be accurately described
by saying that in birds the segmental duct of the kidneys develops as in
Selachians, but that the Müllerian duct develops independently of it.
Since in Selachians the Wolffian duct is equivalent to the segmental duct
of the kidneys with the Müllerian removed from it, when in birds the
Müllerian duct develops independently of the segmental kidney duct, the
latter becomes the same as the Wolffian duct.
The second mode of stating the difference in development in the two cases
represents the embryological facts of the bird far better than the other
method.
It explains why the Wolffian duct appears earlier than the Müllerian and
not at the same time, as one might expect according to the other way of
stating the case. If the Wolffian duct is equivalent to the segmental duct
of Selachians, it must necessarily be the first duct to develop; and not
improbably the development of the Müllerian duct would in birds be expected
to occur at the time corresponding to that at which the primitive duct in
Selachians became split into two ducts.
It probably also explains the similarity in the mode of development of the
Wolffian duct in birds and the primitive duct of the kidneys in Selachians.
This way of stating the case is also in accordance with theoretical
conclusions. As the egg-bearing function of the Müllerian duct became more
and more confirmed we might expect that the adult condition would impress
itself more and more upon the embryonic development, till finally the
Müllerian duct ceased to be at any period connected with the kidneys, and
the history of its origin ceased to be traceable in its development. This
seems to have actually occurred in the higher vertebrates, so that the only
persisting connection between the Müllerian duct and the urinary system is
the brief but important junction of the two at their lower ends on the
sixth or seventh day. This junction justly surprised Waldeyer (_Eierstock
u. Ei_, p. 129), but receives a complete and satisfactory explanation on
the hypothesis given above.
The original development of the segmental tubes is in the bird solely
retained in the tubules of the Wolffian body arising independently of the
Wolffian duct, and I have hitherto failed to find that there is a distinct
division of the Wolffian bodies into segments corresponding with the
vertebral segments.
I have compared the permanent kidneys to the lower portion of the kidneys
of Selachians. The identity of the anatomical condition of the adult
Selachian and embryonic bird which has been already pointed out speaks
strongly in favour of this view; and when we further consider that the duct
of the permanent kidneys is developed in nearly the same way as the
supposed homologous duct in Selachians, the suggested identity gains
further support. The only difficulty is the fact that in Selachians the
tubules of the part of the kidneys under comparison develop as segmental
involutions in point of time anteriorly to their duct, while in birds they
develop in a manner not hitherto certainly made out but apparently in point
of time posteriorly to their duct. But when the immense modifications in
development which the whole of the gland of the excretory organ has
undergone in the bird are considered, I do not think that the fact I have
mentioned can be brought forward as a serious difficulty.
The further points of comparison between the Selachian and the bird are
very simple. The Müllerian duct in its later stages behaves in the higher
vertebrates precisely as in the lower. It becomes in fact the oviduct in
the female and atrophies in the male. The behaviour of the Wolffian duct is
also exactly that of the duct which I have called the Wolffian duct in
Ichthyopsida, and in the tubules of the Wolffian body uniting with the
tubuli seminiferi we have represented the junction of the segmental tubes
with the testis in Selachians and Amphibians. It is probably this junction
of two independent organs which led Waldeyer to the erroneous view that the
tubuli seminiferi were developed from the tubules of the Wolffian body.
With the bird I conclude the history of the origin of the urinogenital
system of vertebrates. I have attempted, and I hope succeeded, in tracing
out by the aid of comparative anatomy and embryology the steps by which a
series of independent and simple segmental organs like those of Annelids
have become converted into the complicated series of glands and ducts which
constitute the urinogenital system of the higher vertebrates. There are no
doubt some points which require further elucidation amongst the Ganoid and
Osseous fishes. The most important points which appear to me still to need
further research, both embryological and anatomical, are the abdominal
pores of fishes, the generative ducts of Ganoids, especially Lepidosteus,
and the generative ducts of Osseous fishes.
The only further point which requires discussion is the embryonic layer
from which these organs are derived.
I have shewn beyond a doubt (_loc. cit._) that in Selachians these organs
are formed from the mesoblast. The unanimous testimony of all the recent
investigators of Amphibians leads to the same conclusion. In birds, on the
other hand, various investigators have attempted to prove that these organs
are derived from the epiblast. The proof they give is the following: the
epiblast and mesoblast appear fused in the region of the axis cord. From
this some investigators have been led to the conclusion that the whole of
the mesoblast is derived from the upper of the two primitive embryonic
layers. To these it may be replied that, even granting their view to be
correct, it is no proof of the derivation of the urinogenital organs from
the epiblast, since it is not till the complete formation of the three
layers that any one of them can be said to exist. Others look upon the
fusion of the two layers as a proof of the passage of cells from the
epiblast into the mesoblast. An assumption in itself, which however is
followed by the further assumption that it is from these epiblast cells
that the urinogenital system is derived! Whatever may have been the
primitive origin of the system, its mesoblastic origin in vertebrates
cannot in my opinion be denied.
Kowalewsky (_Embryo. Stud. an Vermen u. Arthropoda_, Mem. Akad. St
Petersbourg, 1871) finds that the segmental tubes of Annelids develop from
the mesoblast. We must therefore look upon the mesoblastic origin of the
excretory system as having an antiquity greater even than that of
vertebrates.
VIII. ON THE DEVELOPMENT OF THE SPINAL NERVES IN ELASMOBRANCH FISHES[47].
Footnote 47: [From the _Philosophical Transactions of the Royal
Society of London_, Vol. CLXVI. Pt. 1. Received _October 5_,
Read _December 16, 1875_.]
With Plates 22 and 23.
In the course of an inquiry into the development of Elasmobranch Fishes, my
attention has recently been specially directed to the first appearance and
early stages of the spinal nerves, and I have been led to results which
differ so materially from those of former investigators, that I venture at
once to lay them before the Society. I have employed in my investigations
embryos of _Scyllium canicula_, _Scyllium stellare_, _Pristiurus_, and
_Torpedo_. The embryos of the latter animal, especially those hardened in
osmic acid, have proved by far the most favourable for my purpose, though,
as will be seen from the sequel, I have been able to confirm the majority
of my conclusions on embryos of all the above-mentioned genera.
A great part of my work was done at the Zoological Station founded by Dr
Dohrn at Naples; and I have to thank both Dr Dohrn and Dr Eisig for the
uniformly obliging manner in which they have met my requirements for
investigation. I have more recently been able to fill up a number of lacunæ
in my observations by the study of embryos bred in the Brighton Aquarium;
for these I am indebted to the liberality of Mr Lee and the directors of
that institution.
_The first appearance of the Spinal Nerves in Pristiurus._
In a _Pristiurus_-embryo, at the time when two visceral clefts become
visible from the exterior (though there are as yet no openings from without
into the throat), a transverse section through the dorsal region exhibits
the following features (Pl. 22, fig. A):--
The external epiblast is formed of a single row of flattened elongated
cells. Vertically above the neural canal the cells of this layer are more
columnar, and form the rudiment of the primitively continuous dorsal fin.
The neural canal (_nc_) is elliptical in section, and its walls are
composed of oval cells two or three deep. The wall at the two sides is
slightly thicker than at the ventral and dorsal ends, and the cells at the
two ends are also smaller than elsewhere. A typical cell from the side
walls of the canal is about 1/1900 inch in its longest diameter. The
outlines of the cells are for the most part distinctly marked in the
specimens hardened in either chromic or picric acid, but more difficult to
see in those prepared with osmic acid; their protoplasm is clear, and in
the interior of each is an oval nucleus very large in proportion to the
size of its cell. The long diameter of a typical nucleus is about 1/3000
inch, or about two-thirds of that of the cell.
The nuclei are granular, and very often contain several especially large
and deeply stained granules; in other cases only one such is present, which
may then be called a nucleolus.
In sections there may be seen round the exterior of the neural tube a
distinct hyaline membrane: this becomes stained of a brown colour with
osmic acid, and purple or red with hæmatoxylin or carmine respectively.
Whether it is to be looked upon as a distinct membrane differentiated from
the outermost portion of the protoplasm of the cells, or as a layer of
albumen coagulated by the reagents applied, I am unable to decide for
certain. It makes its appearance at a very early period, long before that
now being considered; and similar membranes are present around other organs
as well as the neural tube. The membrane is at this stage perfectly
continuous round the whole exterior of the neural tube _as well on the
dorsal surface as on the ventral_.
The section figured, whose features I am describing, belongs to the middle
of the dorsal region. Anteriorly to this point the spinal cord becomes more
elliptical in section, and the spinal canal more lanceolate; posteriorly,
on the other hand, the spinal canal and tube become more nearly circular in
section. Immediately beneath the neural tube is situated the notochord
(_ch_). It exhibits at this stage a central area rich in protoplasm, and a
peripheral layer very poor in protoplasm; externally it is invested by a
distinct cuticular membrane.
Beneath the notochord is a peculiar rod of cells, constricted from the top
of the alimentary canal[48]. On each side and below this are the two aortæ,
just commencing to be formed, and ventral to these is the alimentary canal.
Footnote 48: Vide Balfour, "Preliminary account of the
Development of Elasmobranch Fishes," _Quart. Journ. of Microsc.
Science_, Oct. 1874, p. 33. [This edition, p. 96.]
On each side of the body two muscle-plates are situated; their upper ends
reach about one-third of the way up the sides of the neural tube. The two
layers which together constitute the muscle-plates are at this stage
perfectly continuous with the somatic and splanchnic layers of the
mesoblast, and the space between the two layers is continuous with the
body-cavity. In addition to the muscle-plates and their ventral
continuations, there are no other mesoblast-cells to be seen. The absence
of all mesoblastic cells dorsal to the superior extremities of the muscles
is deserving of special notice.
Very shortly after this period and, as a rule, before a third visceral
cleft has become visible, the first traces of the spinal nerves make their
appearance.
_First Stage._--The spinal nerves do not appear at the same time along the
whole length of the spinal canal, but are formed first of all in the neck
and subsequently at successive points posterior to this.
Their mode of formation will be most easily understood by referring to
Pl. 22, figs. B I, B II, B III, which are representations of three sections
taken from the same embryo. B I is from the region of the heart; B II
belongs to a part of the body posterior to this, and B III to a still
posterior region.
In most points the sections scarcely differ from Pl. 22, fig. A, which,
indeed, might very well be a posterior section of the embryo to which these
three sections belong.
The chief point, in addition to the formation of the spinal nerves, which
shews the greater age of the embryo from which the sections were taken is
the complete formation of the aortæ.
The upper ends of the muscle-plates have grown no further round the neural
canal than in fig. A, and no scattered mesoblastic connective-tissue cells
are visible.
In fig. A the dorsal surface of the neural canal was as completely rounded
off as the ventral surface; but in fig. B III this has ceased to be the
case. The cells at the dorsal surface of the neural canal have become
rounder and smaller and begun to proliferate, and the uniform outline of
the neural canal has here become broken (fig. B III, _pr_). The peculiar
membrane completely surrounding the canal in fig. A now terminates just
below the point where the proliferation of cells is taking place.
The prominence of cells which springs in this way from the top of the
neural canal is the commencing rudiment of a pair of spinal nerves. In
fig. B II, a section anterior to fig. B III, this formation has advanced
much further (fig. B II, _pr_). From the extreme top of the neural canal
there have now grown out two club-shaped masses of cells, one on each side;
they are perfectly continuous with the cells which form the extreme top of
the neural canal, and necessarily also are in contact with each other
dorsally. Each grows outwards in contact with the walls of the neural
canal; but, except at the point where they take their origin, they are not
continuous with its walls, and are perfectly well separated by a sharp line
from them.
In fig. B I, though the club-shaped processes still retain their attachment
to the summit of the neural canal, they have become much longer and more
conspicuous.
Specimens hardened in both chromic acid (Pl. 22, fig. C) and picric acid
give similar appearances as to the formation of these bodies.
In those hardened in osmic acid, though the mutual relations of the masses
of cells are very clear, yet it is difficult to distinguish the outlines of
the individual cells.
In the chromic acid specimens (fig. C) the cells of these rudiments appear
rounded, and each of them contains a large nucleus.
I have been unable to prepare longitudinal sections of this stage, either
horizontal or vertical, to shew satisfactorily the extreme summit of the
spinal cord; but I would call attention to the fact that the cells forming
the proximal portion of the outgrowth are seen in every transverse section
at this stage, and therefore exist the whole way along, whereas the
_distal_ portion is seen only in every third or fourth section, according
to the thickness of the sections. It may be concluded from this that there
appears a continuous outgrowth from the spinal canal, from which
discontinuous processes grow out.
In specimens of a very much later period (Pl. 23, fig. I) the proximal
portions of the outgrowth are unquestionably continuous with each other,
though their actual junctions with the spinal cord are very limited in
extent. The fact of this continuity at a later period is strongly in favour
of the view that the posterior branches of the spinal nerves arise from the
first as a continuous outgrowth of the spinal cord, from which a series of
distal processes take their origin. I have, however, failed to demonstrate
this point absolutely. The processes, which we may call the
nerve-rudiments, are, as appears from the later stages, equal in number to
the muscle-plates.
It may be pointed out, as must have been gathered from the description
above, that the nerve-rudiments have at this stage but one point of
attachment to the spinal cord, and that this one corresponds with the
dorsal or posterior root of the adult nerve.
The rudiments are, in fact, those of the posterior root only.
The next or second stage in the formation of these structures to which I
would call attention occurs at about the time when three to five visceral
clefts are present. The disappearance from the notochord in the anterior
extremity of the body of a special central area rich in protoplasm serves
as an excellent guide to the commencement of this epoch.
Its investigation is beset with far greater difficulties than the previous
one. This is owing partly to the fact that a number of connective-tissue
cells, which are only with great difficulty to be distinguished from the
cells which compose the spinal nerves, make their appearance around the
latter, and partly to the fact that the attachment of the spinal nerves to
the neural canal becomes much smaller, and therefore more difficult to
study.
Fortunately, however, in _Torpedo_ these peculiar features are not present
to nearly the same extent as in _Pristiurus_ and _Scyllium_.
The connective-tissue cells, though they appear earlier in _Torpedo_ than
in the two other genera, are much less densely packed, and the large
attachment of the nerves to the neural canal is retained for a longer
period.
Under these circumstances I consider it better, before proceeding with this
stage, to give a description of the occurrences in _Torpedo_, and after
that to return to the history of the nerves in the genera _Pristiurus_ and
_Scyllium_.
_The development of the Spinal Nerves in Torpedo._
The youngest _Torpedo_-embryo in which I have found traces of the spinal
nerves belongs to the earliest part of what I called the second stage.
The segmental duct[49] is just appearing, but the cells of the notochord
have not become completely vacuolated. The rudiments of the spinal nerves
extend half of the way towards the ventral side of the spinal cord; they
grow out in a most distinct manner from the dorsal surface of the spinal
cord (Pl. 22, fig. D a, _pr_); but the nerve-rudiments of the two sides are
no longer continuous with each other at the dorsal median line, as in the
earlier _Pristiurus_-embryos. The cells forming the proximal portion of the
rudiment have the same elongated form as the cells of the spinal cord, but
the remaining cells are more circular.
Footnote 49: Vide Balfour, "Origin and History of Urinogenital
Organs of Vertebrates," _Journal of Anatomy and Physiology_,
Oct. 1875. [This edition, No. VII.]
From the summit of the muscle-plates (_mp_) an outgrowth of connective
tissue has made its appearance (_c_), which eventually fills up the space
between the dorsal surface of the cord and the external epiblast. There is
not the slightest difficulty in distinguishing the connective-tissue cells
from the nerve-rudiment. I believe that in this embryo the origin of the
nerves from the neural canal was a continuous one, though naturally the
peripheral ends of the nerve-rudiments were separate from each other.
The most interesting feature of the stage is the commencing formation of
the anterior roots. Each of these arises (Pl. 22, fig. D a, _ar_) as a
small but distinct outgrowth from the epiblast of the spinal cord, near the
ventral corner of which it appears as a conical projection. Even from the
very first it has an indistinct form of termination and a fibrous
appearance, while the protoplasm of which it is composed becomes very
attenuated towards its termination.
The points of origin of the anterior roots from the spinal cord are
separated from each other by considerable intervals. In this fact, and also
in the nerves of the two sides never being united with each other in the
ventral median line, the anterior roots exhibit a marked contrast to the
posterior.
There exists, then, in _Torpedo_-embryos by the end of this stage distinct
rudiments of both the anterior and posterior roots of the spinal nerves.
These rudiments are at first quite independent of and disconnected with
each other, and both take their rise as outgrowths of the epiblast of the
neural canal.
The next _Torpedo_-embryo (Pl. 22, fig. D b), though taken from the same
female, is somewhat older than the one last described. The cells of the
notochord are considerably vacuolated; but the segmental duct is still
without a lumen. The posterior nerve-rudiments are elongated, pear-shaped
bodies of considerable size, and, growing in a ventral direction, have
reached a point nearly opposite the base of the neural canal. They still
remain attached to the top of the neural canal, though the connexion has in
each case become a pedicle so narrow that it can only be observed with
great difficulty.
It is fairly certain that by this stage each posterior nerve-rudiment has
its own separate and independent junction with the spinal cord; their
dorsal extremities are nevertheless probably connected with each other by a
continuous commissure.
The cells composing the rudiments are still round, and have, in fact,
undergone no important modifications since the last stage.
The important feature of the section figured (fig. D b), and one which it
shares with the other sections of the same embryo, is the appearance of
connective-tissue cells around the nerve-rudiment. These cells arise from
two sources; one of these is supplied by the vertebral rudiments, which at
the end of the last stage (Pl. 22, fig. C, _vr_) become split off from the
inner layer of the muscle-plates. The vertebral rudiments have in fact
commenced to grow up on each side of the neural canal, in order to form the
mass of cells out of which the neural arches are subsequently developed.
The dorsal extremities of the muscle-plates form the second source of these
connective-tissue cells. These latter cells lie dorsal and external to the
nerve-rudiments.
The presence of this connective tissue, in addition to the nerve-rudiments,
removes the possibility of erroneous interpretations in the previous stages
of the _Pristiurus_-embryo.
It might be urged that the two masses which I have called nerve-rudiments
are nothing else than mesoblastic connective tissue commencing to develop
around the neural canal, and that the appearance of attachment to the
neural canal which they present is due to bad preparation or imperfect
observation. The sections of both this and the last _Torpedo_-embryo which
I have been describing clearly prove that this is not the case. We have, in
fact, in the same sections the developing connective tissue as well as the
nerve-rudiments, and at a time when the latter still retains its primitive
attachment to the neural canal. The anterior root (fig. D b, _ar_) is still
a distinct conical prominence, but somewhat larger than in the previously
described embryo; it is composed of several cells, and the cells of the
spinal cord in its neighbourhood converge towards its point of origin.
In a _Torpedo_-embryo (Pl. 22, fig. D c) somewhat older than the one last
described, though again derived from the oviduct of the same female, both
the anterior and the posterior rudiments have made considerable steps in
development.
In sections taken from the hinder part of the body I found that the
posterior rudiments nearly agreed in size with those in fig. D b.
It is, however, still less easy than there to trace the junction of the
posterior rudiments with the spinal cord, and the upper ends of the
rudiments of the two sides do not nearly meet.
In a considerable series of sections I failed to find any case in which I
could be absolutely certain that a junction between the nerve and the
spinal cord was effected; and it is possible that in course of the change
of position which this junction undergoes there may be for a short period a
break of continuity between the nerve and the cord. This, however, I do not
think probable. But if it takes place at all, it takes place before the
nerve becomes functionally active, and so cannot be looked upon as
possessing any physiological significance.
The rudiment of the posterior nerve in the hinder portion of the body is
still approximately homogeneous, and no distinction of parts can be found
in it.
In the same region of the body the anterior rudiment retains nearly the
same condition as in the previous stage, though it has somewhat increased
in size.
In the sections taken from the anterior part of the same embryo the
posterior rudiment has both grown in size and also commenced to undergo
histological changes by which it has become divided into a root, a
ganglion, and a nerve.
The root (fig. D c, _pr_) consists of small round cells which lie close to
the spinal cord, and ends dorsally in a rounded extremity.
The ganglion (_g_) consists of larger and more elongated cells, and forms
an oval mass enclosed on the outside by the downward continuation of the
root, having its inner side nearly in contact with the spinal cord.
From its ventral end is continued the nerve, which is of considerable
length, and has a course approximately parallel to that of the
muscle-plate. It forms a continuation of the root rather than of the
ganglion.
Further details in reference to the histology of the nerve-rudiment at this
stage are given later in this paper, in the description of
_Pristiurus_-embryos, of which I have a more complete series of sections
than of the _Torpedo_-embryos.
When compared with the nerve-rudiment in the posterior part of the same
embryo, the nerve-rudiment last described is, in the first place,
considerably larger, and has secondly undergone changes, so that it is
possible to recognize in it parts which can be histologically distinguished
as nerve and ganglion.
The developmental changes which have taken place in the anterior root are
not less important than those in the posterior. The anterior root now forms
a very conspicuous cellular prominence growing out from the ventral corner
of the spinal cord (fig. D c, _ar_). It has a straight course from the
spinal cord to the muscle-plate, and there shews a tendency to turn
downwards at an open angle: this, however, is not represented in the
specimen figured. The cells of which it is composed each contain a large
oval nucleus, and are not unlike the cells which form the posterior
rudiment. The anterior and posterior nerves are still quite unconnected
with each other; and in those sections in which the anterior root is
present the posterior root of the same side is either completely absent or
only a small part is to be seen. The cells of the spinal cord exhibit a
slight tendency to converge towards the origin of the anterior nerve-root.
In the spinal cord itself the epithelium of the central canal is commencing
to become distinguished from the grey matter, but no trace of the white
matter is visible.
I have succeeded in making longitudinal vertical sections of this stage,
which prove that the ends of the posterior roots adjoining the junction
with the cord are all connected with each other (Pl. 22, fig. D d).
If the figure representing a transverse section of the embryo (fig. D c) be
examined, or better still the figure of a section of the slightly older
_Scyllium_-embryo (Pl. 23, fig. H I or I I), the posterior root will be
seen to end dorsally in a rounded extremity, and the junction with the
spinal cord to be effected, not by the extremity of the nerve, but by a
part of it at some little distance from this.
It is from these upper ends of the rudiments beyond the junction with the
spinal cord that I believe the commissures to spring which connect together
the posterior roots.
My sections shewing this for the stage under consideration are not quite as
satisfactory as is desirable; nevertheless they are sufficiently good to
remove all doubt as to the presence of these commissures.
A figure of one of these sections is represented (Pl. 22, fig. D d). In
this figure _pr_ points to the posterior roots and _x_ to the commissures
uniting them.
In a stage somewhat subsequent to this I have succeeded in making
longitudinal sections, which exhibit these junctions with a clearness which
leaves nothing to be desired.
It is there effected (Pl. 23, fig. L) in each case by a protoplasmic
commissure with imbedded nuclei[50]. Near its dorsal extremity each
posterior root dilates, and from the dilated portion is given off on each
side the commissure uniting it with the adjoining roots.
Footnote 50: This commissure is not satisfactorily represented
in the figure. Vide Explanation of Plate 23.
Considering the clearness of this formation in this embryo, as well as in
the embryo belonging to the stage under description, there cannot be much
doubt that at the first formation of the posterior rudiments a continuous
outgrowth arises from the spinal cord, and that only at a later period do
the junctions of the roots with the cord become separated and distinct for
each nerve.
I now return to the more complete series of _Pristiurus_-embryos, the
development of whose spinal nerves I have been able to observe.
_Second Stage of the Spinal Nerves in Pristiurus._
In the youngest of these (Pl. 22, fig. E) the notochord has undergone but
very slight changes, but the segmental duct has made its appearance, and is
as much developed as in the _Torpedo_-embryo from which fig. D b was taken.
(The embryo from which fig. E a was derived had three visceral clefts.)
There have not as yet appeared any connective-tissue cells dorsal to the
top of the muscle-plates, so that the posterior nerve-rudiments are still
quite free and distinct.
The cells composing them are smaller than the cells of the neural canal;
they are round and nucleated; and, indeed, in their histological
constitution the nerve-rudiments exhibit no important deviations from the
previous stage, and they have hardly increased in size. In their mode of
attachment to the neural tube an important change has, however, already
commenced to be visible.
In the previous stage the two nerve-rudiments met above the summit of the
spinal cord and were broadly attached to it there; now their points of
attachment have glided a short distance down the sides of the spinal
cord[51].
Footnote 51: [May 18, 1876.--Observations I have recently made
upon the development of the cranial nerves incline me to adopt
an explanation of the change which takes place in the point of
attachment of the spinal nerves to the cord differing from that
enunciated in the text. I look upon this change as being
apparent rather than real, and as due to a growth of the roof
of the neural canal in the median dorsal line, which tends to
separate the roots of the two sides more and more, and cause
them to assume a more ventral position.]
The two nerve-rudiments have therefore ceased to meet above the summit of
the canal; and in addition to this they appear in section to narrow very
much before becoming united with its walls, so that their junctions with
these appear in a transverse section to be effected by at most one or two
cells, and are, comparatively speaking, very difficult to observe.
In an embryo but slightly older than that represented in Fig. E a the first
rudiment of the anterior root becomes visible. This appears, precisely as
in _Torpedo_, in the form of a small projection from the ventral corner of
the spinal cord (fig. E b, _ar_).
The second step in this stage (Pl. 22, fig. F) is comparable, as far as the
connective-tissue is concerned, with the section of _Torpedo_ (Pl. 22,
fig. D d). The notochord (the histological details of whose
structure are not inserted in this figure) is rather more
developed, and the segmental duct, as was the case with the
corresponding _Torpedo_-embryo, has become hollow at its anterior
extremity.
The embryo from which the section was taken possessed five visceral clefts,
but no trace of external gills.
In the section represented, though from a posterior part of the body, the
dorsal nerve-rudiments have become considerably larger than in the last
embryo; they now extend beyond the base of the neural canal. They are
surrounded to a great extent by mesoblastic tissue, which, as in the case
of the _Torpedo_, takes its origin from two sources, (1) from the
commencing vertebral bodies, (2) from the summits of the muscle-plates.
It is in many cases very difficult, especially with chromic-acid specimens,
to determine with certainty the limits of the rudiments of the posterior
root.
In the best specimens a distinct bordering line can be seen, and it is, as
a rule, possible to state the characters by which the cells of the
nerve-rudiments and vertebral bodies differ. The more important of these
are the following:--(1) The cells of the nerve-rudiment are distinctly
smaller than those of the vertebral rudiment; (2) the cells of the
nerve-rudiment are elongated, and have their long axis arranged parallel to
the long axis of the nerve-rudiment, while the cells surrounding them are
much more nearly circular.
The cells of the nerve-rudiment measure about 1/1600 × 1/4500 to 1/1600 ×
1/3200 inch, those of the vertebral rudiment 1/1600 × 1/1900 inch. The
greater difficulty experienced in distinguishing the nerve-rudiment from
the connective-tissue in _Pristiurus_ than in _Torpedo_ arises from the
fact that the connective-tissue is much looser and less condensed in the
latter than in the former.
The connective-tissue cells which have grown out from the muscle-plates
form a continuous arch over the dorsal surface of the neural tube (vide
Pl. 22, fig. F): and in some specimens it is difficult to see
whether the arch is formed by the rudiment of the posterior root
or by connective-tissue. It is, however, quite easy with the best
specimens to satisfy one's self that it is from the
connective-tissue, and not the nerve-rudiment, that the dorsal
investment of the neural canal is derived.
As in the previous case, the upper ends of each pair of posterior
nerve-rudiments are quite separate from one another, and appear in sections
to be united by a very narrow root to the walls of the neural canal at the
position indicated in fig. F[52].
Footnote 52: The artist has not been very successful in
rendering this figure.
The cells forming the nerve-rudiments have undergone slight modifications;
they are for the most part more distinctly elongated than in the earlier
stage, and appear slightly smaller in comparison with the cells of the
neural canal.
They possess as yet no distinctive characters of nerve-cells. They stain
more deeply with osmic acid than the cells around them, but with
hæmatoxylin there is but a very slight difference in intensity between
their colouring and that of the neighbouring connective-tissue cells.
The anterior roots have grown considerably in length, but their observation
is involved in the same difficulties with chromic-acid specimens as that of
the posterior rudiments.
There is a further difficulty in observing the anterior roots, which arises
from the commencing formation of white matter in the cord. This is present
in all the anterior sections of the embryo from which fig. F is taken. When
the white matter is formed the cells constituting the junction of the
anterior nerve-root with the spinal cord undergo the same changes as the
cells which are being converted into the white matter of the cord, and
become converted into nerve-fibres; these do not stain with hæmatoxylin,
and thus an apparent space is left between the nerve-root and the spinal
cord. This space by careful examination may be seen to be filled up with
fibres. In osmic acid sections, although even in these the white matter is
stained less deeply than the other tissues, it is a matter of comparative
ease to observe the junction between the anterior nerve root and the spinal
cord.
I have been successful in preparing satisfactory longitudinal sections of
embryos somewhat older than that shewn in fig. F, and they bring to light
several important points in reference to the development of the spinal
nerves. Three of these sections are represented in Pl. 22, figs. G1, G2,
and G3.
The sections are approximately horizontal and longitudinal. G1 is the most
dorsal of the three; it is not quite horizontal though nearly longitudinal.
The section passes exactly through the point of attachment of the posterior
roots to the walls of the neural canal.
The posterior rudiments appear as slight prominences of rounded cells
projecting from the wall of the neural canal. From transverse sections the
attachment of the nerves to the wall of the neural canal is proved to be
very narrow, and from these sections it appears to be of some length in the
direction of the long axis of the embryo. A combination of the sections
taken in the two directions leads to the conclusion that the nerves at this
stage thin out like a wedge before joining the spinal cord.
The independent junctions of the posterior rudiments with the spinal cord
at this stage are very clearly shewn, though the rudiments are probably
united with each other just dorsal to their junction with the spinal cord.
The nerves correspond in number with the muscle-plates, and each arises
from the spinal cord, nearly opposite the middle line of the corresponding
muscle-plates (figs. G1 and G2).
Each nerve-rudiment is surrounded by connective-tissue cells, and is
separated from its neighbours by a considerable interval.
At its origin each nerve-rudiment lies opposite the median portion of a
muscle-plate (figs. G1 and G2); but, owing to the muscle-plate acquiring an
oblique direction, at the level of the dorsal surface of the notochord it
appears in horizontal sections more nearly opposite the interval between
two muscle-plates (figs. G2 and G3).
In horizontal sections I find masses of cells which make their appearance
on a level with the ventral surface of the spinal cord. I believe I have in
some sections successfully traced these into the spinal cord, and I have
little doubt that they are the anterior roots of the spinal nerves; they
are opposite the median line of the muscle-plates, and do not appear to
join the posterior roots (vide fig. G3, _ar_).
At the end of this period or second stage the main characters of the spinal
nerves in _Pristiurus_ are the following:--
(1) The posterior nerve-rudiments form somewhat wedge-shaped masses of
tissue attached dorsally to the spinal cord.
(2) The cells of which they are composed are typical undifferentiated
embryonic cells, which can hardly be distinguished from the
connective-tissue cells around them.
(3) The nerves of each pair no longer meet above the summit of the spinal
canal, but are independently attached to its sides.
(4) Their dorsal extremities are probably united by commissures.
(5) The anterior roots have appeared; they form small conical projections
from the ventral corner of the spinal cord, but have no connexion with the
posterior rudiments.
_The Third Stage of the Spinal Nerves in Pristiurus._
With the _third stage_ the first distinct histological differentiations of
the nerve-rudiments commence. Owing to the changes both in the nerves
themselves and in the connective-tissue around them, which becomes less
compact and its cells stellate, the difficulty of distinguishing the nerves
from the surrounding cells vanishes; and the difficulties of investigation
in the later stages are confined to the modes of attachment of the nerves
to the neural canal, and the histological changes which take place in the
rudiments themselves.
The stage may be considered to commence at the period when the external
gills first make their appearance as small buds from the walls of the
visceral clefts. Already, in the earliest rudiments of the posterior root
of this period now figured, a number of distinct parts are visible (Pl. 23,
fig. H I).
Surrounding nearly the whole structure there is present a delicate
investment similar to that which I mentioned as surrounding the neural
canal and other organs; it is quite structureless, but becomes coloured
with all staining reagents. I must again leave open the question whether it
is to be looked upon as a layer of coagulated protoplasm or as a more
definite structure. This investment completely surrounds the proximal
portion of the posterior root, but vanishes near its distal extremity.
The nerve-rudiment itself may be divided into three distinct portions:--(1)
the proximal portion, in which is situated the pedicle of attachment to the
wall of the neural canal; (2) an enlarged portion, which may conveniently,
from its future fate, be called the ganglion; (3) a distal portion beyond
this. The proximal portion presents a fairly uniform diameter, and ends
dorsally in a rounded expansion; it is attached remarkably enough, not by
its extremity, but by its side, to the spinal cord. The dorsal extremities
of the posterior nerves are therefore free; as was before mentioned, they
probably serve as the starting-point of the longitudinal commissures
between the posterior roots.
The spinal cord at this stage is still made up of fairly uniform cells,
which do not differ in any important particulars from the cells which
composed it during the last stage. The outer portion of the most peripheral
layer of cells has already begun to be converted into the white matter.
The delicate investment spoken of before still surrounds the whole spinal
cord, except at the points of junction of the cord with the
nerve-rudiments. Externally to this investment, and separated from it for
the most part by a considerable interval, a mesoblastic sheath (Pl. 23,
fig. H I, _i_) for the spinal cord is beginning to be formed.
The attachment of the nerve-rudiments to the spinal cord, on account of its
smallness, is still very difficult to observe. In many specimens where the
nerve is visible a small prominence may be seen rising up from the spinal
cord at a point corresponding to _x_ (Pl. 23, fig. H I). It is, however,
rare to see this prominence and the nerve continuous with each other: as a
rule they are separated by a slight space, and frequently one of the cells
of the mesoblastic investment of the spinal cord is interposed between the
two. In some especially favourable specimens, similar to the one figured,
there can be seen a distinct cellular prominence (fig. H I, _x_) from the
spinal cord, which becomes continuous with a small prominence on the
lateral border of the nerve-rudiment near its free extremity. The absence
of a junction between the two in a majority of sections is only what might
be expected, considering how minute the junction is.
Owing to the presence of the commissure connecting the posterior roots,
some part of a nerve is present in every section.
The proximal extremity of the nerve-rudiment itself is composed of cells,
which, by their smaller size and a more circular form, are easily
distinguished from cells forming the ganglionic portion of the nerve.
The ganglionic portion of the nerve, by its externally swollen
configuration, is at once recognizable in all the sections in which the
nerve is complete. The delicate investment before mentioned is continuous
around it. The cells forming it are larger and more elongated than the
cells forming the upper portion of the nerve-rudiment: each of them
possesses a large and distinct nucleus.
The remainder of the nerve rudiment forms the commencement of the true
nerve. It can in this stage be traced only for a very small distance, and
gradually fades away, in such a manner that its absolute termination is
very difficult to observe.
The connective-tissue cells which surround the nerve-rudiment are far
looser than in the last stage, and are commencing to throw out processes
and become branched.
The anterior root-nerve has grown very considerable since the last stage.
It projects from the same region of the cord as before, but on approaching
the muscle-plate takes a sudden bend downwards (fig. H II, _ar_).
I have failed to prove that the anterior and posterior roots are at this
stage united.
_Fourth Stage._
In an embryo but slightly more advanced than the one last described,
important steps have been made in the development of the nerve-rudiment.
The spinal cord itself now possesses a covering of white matter; this is
thickest at the ventral portion of the cord, and extends to the region of
the posterior root of the spinal nerve.
The junction of the posterior root with the spinal cord is easier to
observe than in the last stage.
It is still effected by means of unaltered cells, though the cells which
form the projection from the cord to the nerve are commencing to undergo
changes similar to those of the cells which are being converted into white
matter.
In the rudiment of the posterior root itself there are still three distinct
parts, though their arrangement has undergone some alteration and their
distinctness has become more marked (Pl. 23, fig. I I).
The root of the nerve (fig. I I, _pr_) consists, as before, of nearly
circular cells, each containing a nucleus, very large in proportion to the
size of the cell. The cells have a diameter of about 1/3000 of an inch.
This mass forms not only the junction between the ganglion and the spinal
canal, but is also continued into a layer investing the outer side of the
ganglion and continuous with the nerve beyond the ganglion.
The cells which compose the ganglion (fig. I I, _sp.g_) are easily
distinguished from those of the root. Each cell is elongated with an oval
nucleus, large in proportion to the cell; and its protoplasm appears to be
continued into an angular, not to say fibrous process, sometimes at one and
more rarely at both ends. The processes of the cells are at this stage very
difficult to observe: figs. I a, I b, I c represent three cells provided
with them and placed in the positions they occupied in the ganglion.
The relatively very small amount of protoplasm in comparison to the nucleus
is fairly represented in these figures, though not in the drawing of the
ganglion as a whole. In the centre of each nucleus is a nucleolus.
Fig. I b, in which the process points towards the root of the nerve, I
regard as a commencing nerve-fibre: its more elongated shape seems to imply
this. In the next stage special bundles of nerve-fibres become very
conspicuous in the ganglion. The long diameter of an average ganglion-cell
is about 1/1600 of an inch. The whole ganglion forms an oval mass, well
separated both from the nerve-root and the nerve, and is not markedly
continuous with either. On its outer side lies the downward process of the
nerve-root before mentioned.
The nerve itself is still, as in the last case, composed of cells which are
larger and more elongated than either the cells of the root or the
ganglion.
The condition of the anterior root at this stage is hardly altered from
what it was; it is composed of very small cells, which with hæmatoxylin
stain more deeply than any other cell of the section. A figure of it is
given in I II.
Horizontal longitudinal sections of this stage are both easy to make and
very instructive. On Pl. 23, fig. K I is represented a horizontal section
through a plane near the dorsal surface of the spinal cord: each posterior
root is seen in this section to lie nearly opposite the anterior extremity
of a muscle-plate.
In a more ventral plane (fig. K II) this relation is altered, and the
posterior roots lie opposite the hinder parts of the muscle-plates.
The nerves themselves are invested by the hyaline membrane spoken of above;
and surrounding this again there is present a delicate mesoblastic
investment of spindle-shaped cells.
Longitudinal sections also throw light upon the constitution of the
anterior nerve roots (vide fig. K II, _ar_). In the two segments on the
left-hand side in this figure the anterior roots are cut through as they
are proceeding, in a more or less horizontal course, from the spinal cord
to the muscle-plates.
Where the section (which is not quite horizontal) passes through the plane
of the notochord, as on the right-hand side, the anterior roots are cut
transversely. Each root, in fact, changes its direction, and takes a
downward course.
The anterior roots are situated nearly opposite the middle of the
muscle-plates: their section is much smaller than that of the posterior
roots, and with hæmatoxylin they stain more deeply than any of the other
cells in the preparation.
The anterior roots, so far as I have been able to observe, do not at this
stage unite with the posterior; but on this point I do not speak with any
confidence.
The period now arrived at forms a convenient break in the development of
the spinal nerves; and I hope to treat the remainder of the subject,
especially the changes in the ganglion, the development of the
ganglion-cells, and of the nerve-fibres, in a subsequent paper.
I will only add that, not long after the stage last described, the
posterior root unites with the anterior root at a considerable distance
below the cord: this is shewn in Pl. 23, fig. L. Still later the portion of
the root between the ganglion and the spinal cord becomes converted into
nerve-fibres, and the ganglion becomes still further removed from the cord,
while at the same time it appears distinctly divided into two parts.
As regards the development of the cranial nerves, I have made a few
observations, which, though confessedly incomplete, I would desire to
mention here, because, imperfect as they are, they seem to shew that in
Elasmobranch Fishes the cranial nerves resemble the spinal nerves in
arising as outgrowths from the central nervous system.
I have given a figure of the development of a posterior root of a cranial
nerve in fig. M I. The section is taken from the same embryo as figs. B I,
B II, and B III.
It passes through the anterior portion of a thickening of the external
epiblast, which eventually becomes involuted as the auditory vesicle.
The posterior root of a nerve (VII) is seen growing out from the summit of
the hind brain in precisely the same manner that the posterior roots of the
spinal nerves grow out from the spinal cord: it is the rudiment of the
seventh or facial nerve. The section behind this (fig. M II), still in the
region of the ear, has no trace of a nerve, and thus serves to shew the
early discontinuity of the posterior nerve-rudiments which arise from the
brain.
I have as yet failed to detect any cranial anterior roots like those of the
spinal nerves[53]. The similarity in development between the cranial and
spinal nerves is especially interesting, as forming an important addition
to the evidence which at present exists that the cranial nerves are only to
be looked on as spinal nerves, especially modified in connexion with the
changes which the anterior extremity of the body has undergone in existing
vertebrates.
Footnote 53: [May 18, 1876.--Subsequent observations have led
me to the conclusion that no anterior nerve-roots are to be
found in the brain.]
* * * * *
My results may be summarized as follows:--
Along the extreme dorsal summit of the spinal cord there arises on each
side a continuous outgrowth.
From each outgrowth processes corresponding in number to the muscle-plates
grow downwards. These are the posterior nerve-rudiments.
The outgrowths, at first attached to the spinal cord throughout their whole
length, soon cease to be so, and remain in connexion with it in certain
spots only, which form the junctions of the posterior roots with the spinal
cord.
The original outgrowth on each side remains as a bridge, uniting together
the dorsal extremities of all the posterior rudiments. The points of
junction of the posterior roots with the spinal cord are at first situated
at the extreme dorsal summit of the latter, but eventually travel down, and
are finally placed on the sides of the cord.
After these events the posterior nerve-rudiments grow rapidly in size, and
become differentiated into a root (by which they are attached to the spinal
canal), a ganglion, and a nerve.
The anterior roots, like the posterior, are outgrowths from the spinal
cord; but the outgrowths to form them are from the first discontinuous, and
the points from which they originally spring remain as those by which they
are permanently attached to the spinal cord, and do not, as in the case of
the posterior roots, undergo a change of position. The anterior roots
arise, not vertically below, but opposite the intervals between the
posterior roots.
The anterior roots are at first quite separate from the posterior roots;
but soon after the differentiation of the posterior rudiment into a root,
ganglion, and nerve, a junction is effected between each posterior nerve
and the corresponding anterior root. The junction is from the first at some
little distance from the ganglion.
* * * * *
Investigators have hitherto described the spinal nerves as formed from part
of the mesoblast of the protovertebræ. His alone, so far as I know, takes a
different view.
His's[54] observations lead him to the conclusion that the posterior roots
are developed as ingrowths from the external epiblast into the space
between the protovertebræ and the neural canal. These subsequently become
constricted off, unite with the neural canal and form spinal nerves.
Footnote 54: _Erste Anlage des Wirbelthier-Leibes._
These statements, which have not been since confirmed, diverge nearly to
the same extent from my own results as does the ordinary account of the
development of these parts.
Hensen (Virchow's _Archiv_, Vol. XXXI. 1864) also looks upon the spinal
nerves as developed from the epiblast, but not as a direct result of his
own observations[55].
Footnote 55: [May 18, 1876.--Since the above was written
Hensen has succeeded in shewing that in mammals the rudiments
of the posterior roots arise in a manner closely resembling
that described in the present paper; and I have myself, within
the last few days, made observations which incline me to
believe that the same holds good for the chick. My observations
are as yet very incomplete.]
Without attempting, for the present at least, to explain this divergence, I
venture to think that the facts which I have just described have distinct
bearings upon one or two important problems.
One point of general anatomy upon which they throw considerable light is
the primitive origin of nerves.
So long as it was admitted that the spinal and cerebral nerves developed in
the embryo independently of the central nervous system, their mode of
origin always presented to my mind considerable difficulties.
It never appeared clear how it was possible for a state of things to have
arisen in which the central nervous system, as well as the peripheral
terminations of nerves, whether motor or sensory, were formed independently
of each other, while between them a third structure was developed which,
growing in both directions (towards the centre and towards the periphery),
ultimately brought the two into connexion.
That such a condition could be a primitive one seemed scarcely possible.
Still more remarkable did it appear, on the supposition that the primitive
mode of formation of these parts was represented in the developmental
history of vertebrates, that we should find similar structural elements in
the central and in the peripheral nervous systems.
The central nervous system arises from the epiblast, and yet contains
precisely similar nerve-cells and nerve-fibres to the peripheral nervous
system, which, if derived, as is usually stated, from the mesoblast, was
necessarily supposed to have a completely different origin from the central
nervous system.
Both of these difficulties are to a great extent removed by the facts of
the development of these parts in Elasmobranchii.
If it be admitted that the spinal roots develop as outgrowths from the
central nervous system in Elasmobranch Fishes, the question arises, how far
can it be supposed to be possible that in other vertebrates the spinal
roots and ganglia develop independently of the spinal cord, and only
subsequently become united with it.
I have already insisted that this cannot be the primary condition; and
though I am of opinion that the origin of the nerves in higher vertebrates
ought to be worked over again, yet I do not think it impossible that, by a
secondary adaptation, the nerve-roots might develop in the mesoblast[56].
Footnote 56: [May 18, 1876.--Hensen's observations, as well as
those recently made by myself on the chick, render it almost
certain that the nerves in all Vertebrates spring from the
spinal cord.]
The presence of longitudinal commissures connecting the central ends of all
the posterior roots is very peculiar. The commissures may possibly be
looked on as outlying portions of the cord, rather than as parts of the
nerves.
I have not up to this time followed their history beyond a somewhat early
period in embryonic life, and am therefore unacquainted with their fate in
the adult.
As far as I am aware, no trace of similar structures has been met with in
other vertebrates.
The commissures have a very strong resemblance to those by which in
Elasmobranch Fishes the glossopharyngeal nerve and the branches of the
pneumogastric are united in an early embryonic stage[57].
Footnote 57: Balfour, "A Preliminary Account of the
Development of Elasmobranch Fishes," _Q. J. Micros. Sc._ 1874,
plate XV. fig. 14, _v.g._ [This edition, Pl. 4, fig. 14,
_vg_].
I think it not impossible that the commissures in the two cases represent
the same structures. If this is the case, it would seem that the junction
of a number of nerves to form the pneumogastric is not a secondary state,
but the remnant of a primary one, in which all the spinal nerves were
united, as they embryonically are in Elasmobranchii.
One point brought out in my investigations appears to me to have bearings
upon the origin of the central canal of the Vertebrate nervous system, and
in consequence upon the origin of the Vertebrate group itself.
The point I allude to is the posterior nerve-rudiments making their first
appearance at the _extreme dorsal summit_ of the spinal cord.
The transverse section of the ventral nervous cord of an ordinary segmented
worm consists of two symmetrical halves placed side by side.
If by a mechanical folding the two lateral halves of the nervous cord
became bent towards each other, while into the groove formed between the
two the external skin became pushed, we should have an approximation to the
Vertebrate spinal cord. Such a folding might take place to give extra
rigidity to the body in the absence of a vertebral column.
If this folding were then completed in such a way that the groove, lined by
external skin and situated between the two lateral columns of the nervous
system, became converted into a canal, above and below which the two
columns of the nervous system united, we should have in the transformed
nervous cord an organ strongly resembling the spinal cord of Vertebrates.
This resemblance would even extend beyond mere external form. Let the
ventral nervous cord of the common earthworm, _Lumbricus agricola_, be used
for comparison[58], a transverse section of which is represented by
Leydig[59] and Claparède. In this we find that on the ventral surface (the
Annelidan ventral surface) of the nervous cord the ganglion-cells (grey
matter) (_k_) are situated, and on the dorsal side the nerve-fibres or
white matter (_h_). If the folding that I have supposed were to take place,
the grey and white matters would have very nearly the relative situations
which they have in the Vertebrate spinal cord.
Footnote 58: The nervous cords of other Annelids resemble that
of _Lumbricus_ in the relations of the ganglion-cells of the
nerve-fibres.
Footnote 59: _Tafeln zur vergleichenden Anatomie_, Taf. iii.
fig. 8.
The grey matter would be situated in the interior and surround the
epithelium of the central canal, and the white matter would nearly surround
the grey and form the anterior white commissure. The nerves would then
arise, not from the sides of the nervous cord as in existing Vertebrates,
but from its extreme ventral summit.
One of the most striking features which I have brought to light with
reference to the development of the posterior roots, is the fact of their
growing out from the extreme dorsal summit of the neural canal--a position
analogous to the ventral summit of the Annelidan nervous cord. Thus the
posterior roots of the nerves in Elasmobranchii arise in the exact manner
which might have been anticipated were the spinal cord due to such a
folding as I have suggested. The argument from the nerves becomes the
stronger, from the great peculiarity in the position of the outgrowth, a
feature which would be most perplexing without some such explanation as I
have proposed. The central epithelium of the neural canal according to this
view represents the external skin; and its ciliation is to be explained as
a remnant of the ciliation of the external skin now found amongst many of
the lower Annelids.
I have, however, employed the comparison of the Vertebrate and Annelidan
nervous cords, not so much to prove a genetic relation between the two as
to shew the _à priori_ possibility of the formation of a _spinal canal_ and
the _à posteriori_ evidence we have of the Vertebrate spinal canal having
been formed in the way indicated.
I have not made use of what is really the strongest argument for my view,
viz. that the embryonic mode of formation of the spinal canal, by a folding
in of the external epiblast, is the very method by which I have supposed
the spinal canal to have been formed in the ancestors of Vertebrates.
My object has been to suggest a meaning for the peculiar primitive position
of the posterior roots, rather than to attempt to explain in full the
origin of the spinal canal.
EXPLANATION OF THE PLATES[60].
Footnote 60: The figures on these Plates give a fair general
idea of the appearance presented by the developing spinal
nerves; but the finer details of the original drawings have in
several cases become lost in the process of copying.
The figures which are tinted represent sections of embryos
hardened in osmic acid; those without colour sections of
embryos hardened in chromic acid.
PLATE 22.
Fig. A. Section through the dorsal region of an embryo of _Scyllium
stellare_, with the rudiments of two visceral clefts. The section
illustrates the general features at a period anterior to the appearance of
the posterior nerve-roots.
_nc._ neural canal. _mp._ muscle-plate. _ch._ notochord. _x._
subnotochordal rod. _ao._ rudiment of dorsal aorta. _so._ somatopleure.
_sp._ splanchnopleure. _al._ alimentary tract. All the parts of the section
except the spinal cord are drawn somewhat diagrammatically.
Figs. B I, B II, B III. Three sections of a _Pristiurus_-embryo. B I is
through the heart, B II through the anterior part of the dorsal region, and
B III through a point slightly behind this. Drawn with a camera. (Zeiss CC
ocul. 2.)
In B III there is visible a slight proliferation of cells from the dorsal
summit of the neural canal.
In B II this proliferation definitely constitutes two club-shaped masses of
cells (_pr_), both attached to the dorsal summit of the neural canal. The
masses are the rudiments of the posterior nerve-roots.
In B I the rudiments of the posterior roots are of considerable length.
_pr._ rudiment of posterior roots. _nc._ neural canal. _mp._ muscle-plate.
_ch._ notochord. _x._ subnotochordal rod. _ao._ dorsal aorta. _so._
somatopleure. _sp._ splanchnopleure. _al._ alimentary canal. _ht._ heart.
Fig. C. Section from a _Pristiurus_-embryo, slightly older than B. Camera.
(Zeiss CC ocul. 2.) The embryo from which this figure was taken was
slightly distorted in the process of removal from the blastoderm.
_vr._ rudiment of vertebral body. Other reference letters as in previous
figures.
Fig. D a. Section through the dorsal region of a _Torpedo_-embryo with
three visceral clefts. (Zeiss CC ocul. 2.) The section shews the formation
of the dorsal nerve-rudiments (_pr_) and of a ventral anterior
nerve-rudiment (_ar_), which at this early stage is not distinctly
cellular.
_ar._ rudiment of an anterior nerve-root. _y._ cells left behind on the
separation of the external skin from the spinal cord. _c._
connective-tissue cells springing from the summit of the muscle-plates.
Other reference letters as above.
Fig. D b. Section from dorsal region of a _Torpedo_-embryo somewhat older
than D a. Camera. (Zeiss CC ocul. 2.) The posterior nerve-rudiment is
considerably longer than in fig. Da, and its pedicle of attachment to the
spinal cord is thinner. The anterior nerve-rudiment, of which only the edge
is present in the section, is distinctly cellular.
_m._ mesoblast growing up from vertebral rudiment. _sd._ segmental duct.
Fig. D c. Section from a still older _Torpedo_-embryo. Camera. (Zeiss CC
ocul. 2.) The connective-tissue cells are omitted. The rudiment of the
ganglion (_g_) on the posterior root has appeared. The rudiment of the
posterior nerve is much longer than before, and its junction with the
spinal cord is difficult to detect. The anterior root is now an elongated
cellular structure.
_g._ ganglion.
Fig. D d. Longitudinal and vertical section through a _Torpedo_-embryo of
the same age as D c.
The section shews the commissures (_x_) uniting the posterior roots.
Fig. E a. Section of a _Pristiurus_-embryo belonging to the second stage.
Camera. (Zeiss CC ocul. 2.) The section shews the constriction of the
pedicle which attaches the posterior nerve-rudiments to the spinal cord.
_pr._ rudiment of posterior nerve-root. _nc._ neural canal. _mp._
muscle-plate. _vr._ vertebral rudiment. _sd._ segmental duct. _ch._
notochord. _so._ somatopleure. _sp._ splanchnopleure. _ao._ aorta. _al._
alimentary canal.
Fig. E b. Section of a _Pristiurus_-embryo slightly older than Ea. Camera.
(Zeiss CC ocul. 2.) The section shews the formation of the anterior
nerve-root (_ar_).
_ar._ rudiment of the anterior nerve-root.
Fig. F. Section of a _Pristiurus_-embryo with the rudiments of five
visceral clefts. Camera. (Zeiss CC ocul. 2.)
The rudiment of the posterior root is seen surrounded by connective-tissue,
from which it cannot easily be distinguished. The artist has not been very
successful in rendering this figure.
Figs. G1, G2, G3. Three longitudinal and horizontal sections of an embryo
somewhat older than F. The embryo from which these sections were taken was
hardened in osmic acid, but the sections have been represented without
tinting. G1 is most dorsal of the three sections. Camera. (Zeiss CC ocul.
1.)
_nc._ neural canal. _sp.c._ spinal cord. _pr._ rudiment of posterior root.
_ar._ rudiment of anterior root. _mp._ muscle-plate. _c._ connective-tissue
cells. _ch._ notochord.
PLATE 23.
Fig. H I. Section through the dorsal region of a _Pristiurus_-embryo in
which the rudimentary external gills are present as very small knobs.
Camera. (Zeiss CC ocul. 2.)
The section shews the commencing differentiation of the posterior
nerve-rudiment into root (_pr_), ganglion (_sp.g_), and nerve (_n_), and
also the attachment of the nerve-root to the spinal cord (_x_). The
variations in the size and shape of the cells in the different parts of the
nerve-rudiment are completely lost in the figure.
_pr._ posterior nerve-root. _sp.g._ ganglion of posterior root. _n._ nerve
of posterior root. _x._ attachment of posterior root to spinal cord. _w._
white matter of spinal cord. _i._ mesoblastic investment to the spinal
cord.
Fig. H II. Section through the same embryo as H I. (Zeiss CC ocul. 1.)
The section contains an anterior root, which takes its origin at a point
opposite the interval between two posterior roots.
The white matter has not been very satisfactorily represented by the
artist.
Figs. I I, I II. Two sections of a _Pristiurus_-embryo somewhat older than
H. Camera. (Zeiss CC ocul. 1.)
The connective-tissue cells are omitted.
Figs. I a, I b, I c. Three isolated cells from the ganglion of one of the
posterior roots of the same embryo.
Figs. K I, K II. Two horizontal longitudinal sections through an embryo in
which the external gills have just appeared. K I is the most dorsal of the
two sections. Camera. (Zeiss CC ocul. 1.)
The sections shew the relative positions of the anterior and posterior
roots at different levels.
_pr._ posterior nerve-rudiment. _ar._ anterior nerve-rudiment. _sp.c._
spinal cord. _n.c._ neural canal. _mp._ muscle-plate. _mp´._ first-formed
muscles.
Fig. L. Longitudinal and vertical section through the trunk of a
_Scyllium_-embryo after the external gills have attained their full
development. Camera. (Zeiss CC ocul. 1.)
The embryo was hardened in a mixture of chromic acid and osmic acid.
The section shews the commissures which dorsally unite the posterior roots,
and also the junction of the anterior and posterior roots. The commissures
are unfortunately not represented in the figure with great accuracy; their
outlines are in nature perfectly regular, and not, as in the figure,
notched at the junctions of the cells composing them. Their cells are
apparently more or less completely fused, and certainly not nearly so
clearly marked as in the figure. The commissures stain very deeply with the
mixture of osmic and chromic acid, and form one of the most conspicuous
features in successful longitudinal sections of embryos so hardened. In
sections hardened with chromic acid only they cannot be seen with the same
facility.
_sp.c._ spinal cord. _gr._ grey matter. _w._ white matter. _ar._ anterior
root. _pr._ posterior root. _x._ commissure uniting the posterior roots.
Figs. M I, M II. Two sections through the head of the same embryo as
fig. B. M I, the foremost of the two, passes through the anterior part of
the thickening of epiblast, which becomes involuted as the auditory
vesicle. It contains the rudiment of the seventh nerve, VII. Camera.
(Zeiss CC ocul. 2.)
VII. rudiment of seventh nerve. _au._ thickening of external epiblast,
which becomes involuted as the auditory vesicle. _n.c._ neural canal. _ch._
notochord. _pp._ body-cavity in the head. _so._ somatopleure. _sp._
splanchnopleure. _al._ throat exhibiting an outgrowth to form the first
visceral cleft.
IX. ON THE SPINAL NERVES OF AMPHIOXUS[61].
Footnote 61: From the _Journal of Anatomy and Physiology_,
Vol. X. 1876.
During a short visit to Naples in January last, I was enabled, through the
kindness of Dr Dohrn, to make some observations on the spinal nerves of
Amphioxus. These were commenced solely with the view of confirming the
statements of Stieda on the anatomy of the spinal nerves, which, if
correct, appeared to me to be of interest in connection with the
observations I had made that, in Elasmobranchii, the anterior and posterior
roots arise alternately and not in the same vertical plane. I have been led
to conclusions on many points entirely opposed to those of Stieda, but,
before recording these, I shall proceed briefly to state his results, and
to examine how far they have been corroborated by subsequent observers.
Stieda[62], from an examination of sections and isolated spinal cords, has
been led to the conclusion that, in Amphioxus, the nerves of the opposite
sides arise alternately, except in the most anterior part of the body,
where they arise opposite each other. He also states that the nerves of the
same side issue alternately from the dorsal and ventral corners of the
spinal cord. He regards two of these roots (dorsal and ventral) on the same
side as together equivalent to a single spinal nerve of higher vertebrates
formed by the coalescence of a dorsal and ventral root.
Footnote 62: _Mém. Acad. Pétersbourg_, Vol. XIX.
Langerhans[63] apparently agrees with Stieda as to the facts about the
alternation of dorsal and ventral roots, but differs from him as to the
conclusions to be drawn from those facts. He does not, for two reasons,
believe that two nerves of Amphioxus can be equivalent to a single nerve in
higher vertebrates: (1) Because he finds no connecting branch between two
succeeding nerves, and no trace of an anastomosis. (2) Because he finds
that each nerve in Amphioxus supplies a complete myotome, and he considers
it inadmissible to regard the nerves, which in Amphioxus together supply
_two myotomes_, as equivalent to those which in higher vertebrates supply a
_single myotome only_.
Footnote 63: _Archiv f. mikr. Anatomie_, Vol. XII.
Although the agreement as to facts between Langerhans and Stieda is
apparently a complete one, yet a critical examination of the statements of
these two authors proves that their results, on one important point at
least, are absolutely contradictory. Stieda, Pl. III. fig. 19, represents a
longitudinal and horizontal section through the spinal cord which exhibits
the nerves arising alternately on the two sides, and represents each
myotome supplied by _one nerve_. In his explanation of the figure he
expressly states that the nerves of one plane only (_i.e._ only those with
dorsal or only those with ventral roots) are represented; so that if all
the nerves which issue from the spinal cord had been represented double the
number figured must have been present. But since each myotome is supplied
by _one_ nerve in the figure, if all the nerves present were represented,
each myotome would be supplied by two nerves.
Since Langerhans most emphatically states that only _one nerve_ is present
for _each myotome_, it necessarily follows that he or Stieda has made an
important error; and it is not too much to say that this error is more than
sufficient to counterbalance the value of Langerhans' evidence as a
confirmation of Stieda's statements.
I commenced my investigations by completely isolating the nervous system of
Amphioxus by maceration in nitric acid according to the method recommended
by Langerhans[64]. On examining specimens so obtained it appeared that, for
the greater length of the cord, the nerves arose alternately on the two
sides, as was first stated by Owsjannikow, and subsequently by Stieda and
Langerhans; but to my surprise not a trace could be seen of a difference of
level in the origin of the nerves of the same side.
Footnote 64: _Loc. cit._
The more carefully the specimens were examined from all points of view, the
more certainly was the conclusion forced upon me, that nerves issuing from
the ventral corner of the spinal cord, as described by Stieda, had no
existence.
Not satisfied by this examination, I also tested the point by means of
sections. I carefully made transverse sections of a successfully hardened
Amphioxus, through the whole length of the body. There was no difficulty in
seeing the dorsal roots in every third section or so, but not a trace of a
ventral root was to be seen. There can, I think, be no doubt, that, had
ventral roots been present, they must, in some cases at least, have been
visible in my sections.
In dealing with questions of this kind it is no doubt difficult to prove a
negative; but, since the two methods of investigation employed by me both
lead to the same result, I am able to state with considerable confidence
that my observations lend no support to the view that the alternate spinal
nerves of Amphioxus have their roots attached to the ventral corner of the
spinal cord.
How a mistake on this point arose it is not easy to say. All who have
worked with Amphioxus must be aware how difficult it is to conserve the
animal in a satisfactory state for making sections. The spinal cord,
especially, is apt to be distorted in shape, and one of its ventral corners
is frequently produced into a horn-like projection terminating in close
contact with the sheath. In such cases the connective tissue fibres of the
sheath frequently present the appearance of a nerve-like prolongation of
the cord; and for such they might be mistaken if the sections were examined
in a superficial manner. It is not, however, easy to believe that, with
well conserved specimens, a mistake could be made on this point by so
careful and able an investigator as Stieda, especially considering that the
histological structure of the spinal nerves is very different from that of
the fibrous prolongations of the sheath of the spinal cord.
It only remains for me to suppose that the specimens which Stieda had at
his disposal, were so shrunk as to render the origin of the nerves very
difficult to determine.
The arrangement of the nerves of Amphioxus, according to my own
observations, is as follows.
The anterior end of the central nervous system presents on its left and
dorsal side a small pointed projection, into which is prolonged a
diverticulum from the dilated anterior ventricle of the brain. This may
perhaps be called the olfactory nerve, though clearly of a different
character to the other nerves. It was first accurately described by
Langerhans[65].
Footnote 65: _Loc. cit._
Vertically below the olfactory nerve there arise two nerves, which issue at
the same level from the ventral side of the anterior extremity of the
central nervous system. These form the first pair of nerves, and are the
only pair which arise from the ventral portion of the cerebro-spinal cord.
The two nerves, which form the second pair, arise also opposite each other
but from the dorsal side of the cord. The first and second pair of nerves
have both been accurately drawn and described by Langerhans: they, together
with the olfactory nerve, can easily be seen in nervous systems which have
been isolated by maceration.
In the case of the third pair of nerves, the nerve on the right-hand side
is situated not quite opposite but slightly behind that on the left. The
right nerve of the fourth pair is situated still more behind the left, and,
in the case of the fifth pair, the nerve to the right is situated so far
behind the left nerve that it occupies a position half-way between the left
nerves of the fifth and sixth pairs. In all succeeding nerves the same
arrangement holds good, so that they exactly alternate on two sides.
Such is the arrangement carefully determined by me from one specimen. It is
possible that it may not be absolutely constant, but the following general
statement almost certainly holds good.
All the nerves of Amphioxus, except the first pair, have their roots
inserted in the dorsal part of the cord. In the case of the first two pairs
the nerves of the two sides arise opposite each other; in the next few
pairs, the nerves on the right-hand side gradually shift backwards: the
remaining nerves spring alternately from the two sides of the cord.
For each myotome there is a single nerve, which enters, as in the case of
other fishes, the intermuscular septum. This point may easily be determined
by means of longitudinal sections, or less easily from an examination of
macerated specimens. I agree with Langerhans in denying the existence of
ganglia on the roots of the nerves.
X.
A MONOGRAPH
ON THE
DEVELOPMENT OF
ELASMOBRANCH FISHES.
PUBLISHED 1878.
PREFACE.
The present Monograph is a reprint of a series of papers published in the
_Journal of Anatomy and Physiology_ during the years 1876, 1877 and 1878.
The successive parts were struck off as they appeared, so that the earlier
pages of the work were in print fully two years ago. I trust the reader
will find in this fact a sufficient excuse for a certain want of coherence,
which is I fear observable, as well as for the omission of references to
several recent publications. The first and second chapters would not have
appeared in their present form had I been acquainted, at the time of
writing them, with the researches which have since been published, on the
behaviour of the germinal vesicle and on the division of nuclei. I may also
call attention to the valuable papers of Prof. His[66] on the formation of
the layers in Elasmobranchii, and of Prof. Kowalevsky[67] on the
development of Amphioxus, to both of which I would certainly have referred,
had it been possible for me to do so.
Footnote 66: _Zeitschrift f. Anat. u. Entwicklungsgeschichte_,
Bd. II.
Footnote 67: _Archiv f. Micr. Anat._ Bd. XIII.
Professor His deals mainly with the subjects treated of in Chapter III.,
and gives a description very similar to my own of the early stages of
development. His interpretations of the observed changes are, however, very
different from those at which I have arrived. Although this is not the
place for a discussion of Prof. His's views, I may perhaps state that, in
spite of the arguments he has brought forward in support of his position, I
am still inclined to maintain the accuracy of my original account. The very
striking paper on Amphioxus by Kowalevsky (the substance of which I
understand to have been published in Russia at an earlier period) contains
a confirmation of the views expressed in chapter VI. on the development of
the mesoblast, and must be regarded as affording a conclusive
demonstration, that in the case of Vertebrata the mesoblast has primitively
the form of a pair of diverticula from the walls of the archenteron.
* * * * *
The present Memoir, while differing essentially in scope and object from
the two important treatises by Professors His[68] and Götte[69], which have
recently appeared in Germany, has this much in common with them, that it
deals monographically with the development of a single type: but here the
resemblance ends. Both of these authors seek to establish, by a careful
investigation of the development of a single species, the general plan of
development of Vertebrates in general, if not of the whole animal kingdom.
Both reject the theory of descent, as propounded by Mr Darwin, and offer
completely fresh explanations of the phenomena of Embryology. Accepting, as
I do, the principle of natural selection, I have had before me, in writing
the Monograph, no such ambitious aim as the establishment of a completely
new system of Morphology. My object will have been fully attained if I have
succeeded in adding a few stones to the edifice, the foundations of which
were laid by Mr Darwin in his work on the _Origin of Species_.
Footnote 68: _Erste Anlage des Wirbelthierleibes._
Footnote 69: _Entwicklungsgeschichte der Unke._
I may perhaps call attention to one or two special points in this work
which seem to give promise of further results. The chapter on the
Development of the Spinal and Cranial Nerves contains a modification of the
previously accepted views on this subject, which may perhaps lead to a more
satisfactory conception of the origin of nerves than has before been
possible, and a more accurate account of the origin of the muscle-plates
and vertebral column. The attempt to employ the embryological relations of
the cephalic prolongations of the body-cavity, and of the cranial nerves,
in the solution of the difficult problems of the Morphology of the head,
may prove of use in the line of study so successfully cultivated by our
great English Anatomist, Professor Huxley. Lastly, I venture to hope that
my conclusions in reference to the relations of the sympathetic system and
the suprarenal body, and to the development of the mesoblast, the
notochord, the limbs, the heart, the venous system, and the excretory
organs, are not unworthy of the attention of Morphologists.
* * * * *
The masterly manner in which the systematic position of Elasmobranchii is
discussed by Professor Gegenbaur, in the introduction to his Monograph on
the Cranial Skeleton of the group, relieves me from the necessity of
entering upon this complicated question. It is sufficient for my purpose
that the Elasmobranch Fishes be regarded as forming one of the most
primitive groups among Vertebrates, a view which finds ample confirmation
in the importance of the results to which Prof. Gegenbaur and his pupils
have been led in this branch of their investigations.
* * * * *
Though I trust that the necessary references to previous contributions in
the same department of enquiry have not been omitted, the 'literature of
the subject' will nevertheless be found to occupy a far smaller share of
space than is usual in works of a similar character. This is an intentional
protest on my part against, what appears to me, the unreasonable amount of
space so frequently occupied in this way. The pages devoted to the
'previous literature' only weary the reader, who is not wise enough to skip
them, and involve a great and useless expenditure of time on the part of
any writer, who is capable of something better than the compilation of
abstracts.
* * * * *
In conclusion, my best thanks are due to Drs Dohrn and Eisig for the
uniformly kind manner in which they have forwarded my researches both at
the Zoological Station in Naples, and after my return to England; and also
to Mr Henry Lee and to the Manager and Directors of the Brighton Aquarium,
who have always been ready to respond to my numerous demands on their
liberality.
To my friend and former teacher Dr Michael Foster I tender my sincerest
thanks for the never-failing advice and assistance which he has given
throughout the whole course of the work.
TABLE OF CONTENTS.
CHAPTER I.
THE RIPE OVARIAN OVUM, pp. 213-221.
Structure of ripe ovum. Atrophy of germinal vesicle. The extrusion of its
membrane and absorption of its contents. Oellacher's observations on the
germinal vesicle. Götte's observations. Kleinenberg's observations. General
conclusions on the fate of the germinal vesicle. Germinal disc.
CHAPTER II.
THE SEGMENTATION, pp. 222-245.
Appearance of impregnated germinal disc. Stage with two furrows. Stage with
twenty-one segments. Structure of the sides of the furrows. Later stages of
segmentation. Spindle-shaped nuclei. Their presence outside the blastoderm.
Knobbed nuclei. Division of nuclei. Conclusion of segmentation. Nuclei of
the yolk. Asymmetry of the segmented blastoderm. Comparison of Elasmobranch
segmentation with that of other meroblastic ova. Literature of Elasmobranch
segmentation.
CHAPTER III.
FORMATION OF THE LAYERS, pp. 246-285.
Division of blastoderm into two layers. Formation of segmentation cavity.
Disappearance of cells from floor of segmentation cavity. Nuclei of yolk
and of blastoderm. Formation of embryonic rim. Appearance of a layer of
cells on the floor of the segmentation cavity. Formation of mesoblast.
Formation of medullary groove. Disappearance of segmentation cavity.
Comparison of segmentation cavity of Elasmobranchii with that of other
types. Alimentary cavity. Formation of mesoblast in two lateral plates.
Protoplasmic network of yolk. Summary. Nature of meroblastic ova.
Comparison of Elasmobranch development with that of other types. Its
relation to the Gastrula. Haeckel's views on vertebrate Gastrula. Their
untenable nature. Comparison of primitive streak with blastopore.
Literature.
CHAPTER IV.
GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO AT SUCCESSIVE
STAGES, pp. 286-297.
Description of Stages A-Q. Enclosure of yolk by blastoderm. Relation of the
anus of Rusconi to the blastopore.
CHAPTER V.
STAGES B-G, pp. 298-314.
_General features of the epiblast._--Original uniform constitution.
Separation into lateral and central portions. _The medullary groove._--Its
conversion into the medullary canal. _The mesoblast._--Its division into
somatic and splanchnic layers. Formation of protovertebræ. The lateral
plates. The caudal swellings. The formation of the body-cavity in the head.
_The alimentary canal._--Its primitive constitution. The anus of Rusconi.
Floor formed by yolk. Formation of cellular floor from cells formed around
nuclei of the yolk. Communication behind of neural and alimentary canals.
Its discovery by Kowalevsky. Its occurrence in other instances. _General
features of the hypoblast._ _The notochord._--Its formation as a median
thickening of the hypoblast. Possible interpretations to be put on this.
Its occurrence in other instances.
CHAPTER VI.
DEVELOPMENT OF THE TRUNK DURING STAGES G TO K, pp. 315-360.
Order of treatment. _External epiblast._--Characters of epiblast. Its late
division into horny and epidermic layers. Comparison of with Amphibian
epiblast. _The unpaired fins._ _The paired fins._--Their formation as
lateral ridges of epiblast. Hypothesis that the limbs are remnants of
continuous lateral fins. _Mesoblast._--Constitution of lateral plates of
mesoblast. Their splanchnic and somatic layers. Body-cavity constituting
space between them. Their division into lateral and vertebral plates.
Continuation of body-cavity into vertebral plates. Protovertebræ. Division
into muscle-plates and vertebral bodies. Development of muscle-plates.
Disappearance of segmentation in tissue to form vertebral bodies.
Body-cavity and parietal plates. Primitive independent halves of
body-cavity. Their ventral fusion. Separation of anterior part of
body-cavity as pericardial cavity. Communication of pericardial and
peritoneal cavities. Somatopleure and splanchnopleure. _Résumé._ General
considerations on development of mesoblast. Probability of lateral plates
of mesoblast in Elasmobranchii representing alimentary diverticula. Meaning
of secondary segmentation of vertebral column. The _urinogenital
system._--Development of segmental duct and segmental tubes as solid
bodies. Formation of a lumen in them, and their opening into body-cavity.
Comparison of segmental duct and segmental tubes. Primitive ova. Their
position. Their structure. _The notochord._--The formation of its sheath.
The changes in its cells.
CHAPTER VII.
GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE K TO THE
CLOSE OF EMBRYONIC LIFE, pp. 361-377.
_External epiblast._--Division into separate layers. Placoid scales.
Formation of their enamel. _Lateral line._--Previous investigations.
Distinctness of lateral line and lateral nerve. Lateral nerve a branch of
vagus. Lateral line a thickening of epiblast. Its greater width behind. Its
conversion into a canal by its cells assuming a tubular arrangement. The
formation of its segmental apertures. Mucous canals of the head. Their
nerve-supply. Reasons for dissenting from Semper's and Götte's view of
lateral nerve. _Muscle-plates._--Their growth. Conversion of both layers
into muscles. Division into dorso-lateral and ventro-lateral sections.
Derivation of limb-muscles from muscle-plates. _Vertebral column and
notochord._--Previous investigations. Formation of arches. Formation of
cartilaginous sheath of notochord and membrana elastica externa.
Differentiation of neural arches. Differentiation of hæmal arches.
Segmentation of cartilaginous sheath of notochord. Vertebral and
intervertebral regions. Notochord.
CHAPTER VIII.
DEVELOPMENT OF THE SPINAL NERVES AND OF THE SYMPATHETIC
NERVOUS SYSTEM, pp. 378-396.
_The spinal nerves._--Formation of posterior roots. Later formation of
anterior roots. Development of commissure uniting posterior roots.
Subsequent development of posterior roots. Their change in position.
Development of ganglion. Further changes in anterior roots. Junction of
anterior and posterior roots. Summary. _General considerations._--Origin of
nerves. Hypothesis explaining peripheral growth. Hensen's views. Later
investigations. Götte. Calberla. Relations between Annelidan and Vertebrate
nervous systems. Spinal canal. Dr Dohrn's views. Their difficulties.
Hypothesis of dorsal coalescence of lateral nerve cords. _Sympathetic
nervous system._--Development of sympathetic ganglia on branches of spinal
nerves. Formation of sympathetic commissure.
CHAPTER IX.
DEVELOPMENT OF THE ORGANS IN THE HEAD, pp. 397-445.
DEVELOPMENT OF THE BRAIN, pp. 397-407. General history.
_Fore-brain._--Optic vesicles. Infundibulum. Pineal gland. Olfactory lobes.
Lateral ventricles. _Mid-brain._ _Hind-brain._--Cerebellum.
Medulla.--Previous investigations. Huxley. Miklucho-Maclay. Wilder. ORGANS
OF SENSE, pp. 407-412. _Olfactory organ._--Olfactory pit. Schneiderian
folds. _Eye._ General development. Hyaloid membrane. Lens capsule.
Processus falciformis. _Auditory organs._--Auditory pit. Semicircular
canals. MOUTH INVOLUTION and PITUITARY BODY, pp. 412-414. Outgrowth of
pituitary involution. Separation of pituitary sack. Junction with
infundibulum. DEVELOPMENT OF CRANIAL NERVES, pp. 414-428. Early development
of 5th, 7th, 8th, 9th and 10th cranial nerves. Distribution of the nerves
in the adult. _The fifth nerve._--Its division into ophthalmic and
mandibular branches. Later formation of superior maxillary branch. _Seventh
and auditory nerves._--Separation of single rudiment into seventh and
auditory. Forking of seventh nerve over hyomandibular cleft. Formation of
anterior branch to form ramus ophthalmicus superficialis of adult. General
view of morphology of branches of seventh nerve. _Glossopharyngeal and
vagus nerves._--General distribution at stage L. Their connection by a
commissure. Junction of the commissure with commissure connecting posterior
roots of spinal nerves. Absence of anterior roots. Hypoglossal nerve.
MESOBLAST OF HEAD, pp. 429-432. _Body-cavity and myotomes of
head._--Continuation of body-cavity into head. Its division into segments.
Development of muscles from their walls. General mesoblast of head.
NOTOCHORD IN HEAD, p. 433. HYPOBLAST OF THE HEAD, pp. 433-434. The
formation of the gill-slits. Layer from which gills are derived.
SEGMENTATION OF THE HEAD, pp. 434-440. Indication of segmentation afforded
by (1) cranial nerves, (2) visceral clefts, (3) head-cavities. Comparison
of results obtained.
CHAPTER X.
THE ALIMENTARY CANAL, pp. 446-459.
_The solid oesophagus._--OEsophagus originally hollow. Becomes solid during
Stage K. _The postanal section of the alimentary tract._--Continuity of
neural and alimentary canals. Its discovery by Kowalevsky. The postanal
section of gut. Its history in Scyllium. Its disappearance. _The cloaca and
anus._--The formation of the cloaca. Its junction with segmental ducts.
Abdominal pockets. Anus. _The thyroid body._--Its formation in region of
mandibular arch. It becomes solid. Previous investigations. _The
pancreas._--Arises as diverticulum from dorsal side of duodenum. Its
further growth. Formation of duct. _The liver._--Arises as ventral
diverticulum of duodenum. Hepatic cylinders. Comparison with other types.
_The subnotochordal rod._--Its separation from dorsal wall of alimentary
canal. The section of it in the trunk. In the head. Its disappearance.
Views as to its meaning.
CHAPTER XI.
THE VASCULAR SYSTEM AND VASCULAR GLANDS, pp. 460-478.
_The heart._--Its development. Comparison with other types. Meaning of
double formation of heart. _The general circulation._ The venous system.
The primitive condition of. Comparison of, with Amphioxus and Annelids. The
cardinal veins. Relations of caudal vein. _The circulation of the
yolk-sack._--Previous observations. Various stages. Difference of type in
amniotic Vertebrates. _The vascular glands._--Suprarenal and interrenal
bodies. Previous investigations. _The suprarenal bodies._--Their structure
in the adult. Their development from the sympathetic ganglia. _The
interrenal body._--Its structure in the adult. Its independence of
suprarenal bodies. Its development.
CHAPTER XII.
THE ORGANS OF EXCRETION, pp. 479-520.
Previous investigations. _Excretory organs and genital ducts in adult._ _In
male._--Kidney and Wolffian body. Wolffian duct. Ureters. Cloaca. Seminal
bladders. Rudimentary oviduct. _In female._--Wolffian duct. Ureters.
Cloaca.--Segmental openings. Glandular tubuli of kidney. Malpighian bodies.
Accessory Malpighian bodies. Relations of to segmental tubes. Vasa
efferentia. Comparison of Scyllium with other Elasmobranchii. _Development
of segmental tubes._ Their junction with segmental duct. Their division
into four segments. Formation of Malpighian bodies. Connection between
successive segments. Morphological interest of. _Development of Müllerian
and Wolffian ducts._ _In female_--General account. Formation of oviduct as
nearly solid cord. Hymen. _In male_--Rudimentary Müllerian
duct.--Comparison of development of Müllerian duct in Birds and
Elasmobranchii. Own researches. Urinal cloaca. _Formation of Wolffian body
and kidney proper._--General account. Details of formation of ureters.
_Vasa efferentia._--Views of Semper and Spengel. Difficulties of Semper's
views. Unsatisfactory result of own researches. General homologies.
_Résumé._ Postscript.
CHAPTER I.
THE RIPE OVARIAN OVUM.
The ripe ovum is nearly spherical, and, after the removal of its capsule,
is found to be unprovided with any form of protecting membrane.
My investigations on the histology of the ripe ovarian ovum have been made
with the ova of the Gray Skate (_Raja batis_) only, and owing to a
deficiency of material are somewhat imperfect.
The bulk of the ovum is composed of yolk spherules, imbedded in a
protoplasmic matrix. Dr Alexander Schultz[70], who has studied with great
care the constitution of the yolk, finds, near the centre of the ovum, a
kernel of small yolk spherules, which is succeeded by a zone of spherules
which gradually increase in size as they approach the surface. But, near
the surface, he finds a layer in which they again diminish in size and
exhibit numerous transitional forms on the way to molecular yolk granules.
These Dr Schultz regards as in a retrogressive condition.
Footnote 70: _Archiv für Micro. Anat._ Vol. XI. 1875.
Another interesting feature about the yolk is the presence in it of a
protoplasmic network. Dr Schultz has completely confirmed, and on some
points enlarged, my previous observations on this subject[71]. Dr Schultz's
confirmation is the more important, since he appears to be unacquainted
with my previous investigations. In my paper (_loc. cit._), after giving a
description of the network I make the following statement as to its
distribution.
"A specimen of this kind is represented in Plate 14, fig. 2,
_n.y_, where the meshes of the network are seen to be finer
immediately around the nuclei, and coarser in the intervals.
The specimen further shews, in the clearest manner, that
this network is not divided into areas, each representing a
cell and each containing a nucleus. I do not know to what
extent this network extends into the yolk. I have never yet
seen the limits of it, though it is very common to see the
coarsest yolk-granules lying in its meshes. Some of these
are shewn in Plate 14, fig. 2, _y.k._" [This edition, p.
65.]
Footnote 71: _Quart. Journ. Micro. Science_, Oct. 1874. [This
edition, No. V.]
Dr Schultz, by employing special methods of hardening and cutting sections
of the whole egg, has been able to shew that this network extends, in the
form of fine radial lines, from the centre to the circumference; and he
rightly states, that it exhibits no cell-like structures. I have detected
this network extending throughout the whole yolk in young eggs, but have
failed to see it with the distinctness which Dr Schultz attributes to it in
the ripe ovum. Since it is my intention to enter fully both into the
structure and meaning of this network in my account of a later stage, I say
no more about it here.
At one pole of the ripe ovum a slight examination demonstrates the presence
of a small circular spot, sharply distinguished from the remainder of the
yolk by its lighter colour. Around this spot is an area which is also of a
lighter colour than the yolk, and the outer border of which gradually
shades into the normal tint of the yolk. If a section be made through this
part (vide Pl. 6, fig. 1) the circular spot will be found to be the
germinal vesicle, and the area around it a disc of yolk containing smaller
spherules than the surrounding parts. The germinal vesicle possessed the
same structure in both the ripe eggs examined by me; and, in both, it was
situated quite on the external surface of the yolk.
In one of my specimens it was flat above, but convex below; in the other
and, on the whole, the better preserved of the two, it had the somewhat
quadrangular but rather irregular section represented in Pl. 6, fig. 1. It
consisted of a thickish membrane and its primitive contents. The membrane
surrounded the upper part of the contents and exhibited numerous folds and
creases (vide fig. 1). As it extended downwards it became thinner, and
completely disappeared at some little distance from the lower end of the
contents. These, therefore, rested below on the yolk. At its circumference
the membrane of the disc was produced into a kind of fold, forming a rim
which rested on the surface of the yolk.
In neither of my specimens is the cavity in the upper part of the membrane
filled by the contents; and the upper part of the membrane is so folded and
creased that sections through almost any portion of it pass through the
folds. The regularity of the surface of the yolk is not broken by the
germinal vesicle, and the yolk around exhibits not the slightest signs of
displacement. In the germinal vesicle figured the contents are somewhat
irregular in shape; but in my other specimen they form a regular mass
concave above and convex below. In both cases they rest on the yolk, and
the floor of the yolk is exactly moulded to suit the surface of the
contents of the germinal vesicle. The contents have a granular aspect, but
differ in constitution from the surrounding yolk. Each germinal vesicle
measured about one-fiftieth of an inch in diameter.
It does not appear to me possible to suppose that the peculiar appearances
which I have drawn and described are to be looked upon as artificial
products either of the chromic acid, in which the ova were hardened, or of
the instrument with which sections of them were made. It is hardly
conceivable that chromic acid could cause a rupture of the membrane and the
ejection of the contents of the vesicle. At the same time the uniformity of
the appearances in the different sections, the regularity of the whole
outline of the egg, and the absence of any signs of disturbance in the
yolk, render it impossible to believe that the structures described are due
to faults of manipulation during or before the cutting of the sections.
We can only therefore conclude that they represent the real state of the
germinal vesicle at this period. No doubt they alone do not supply a
sufficient basis for any firm conclusions as to the fate of the germinal
vesicle. Still, if they cannot sustain, they unquestionably support certain
views. The natural interpretation of them is that the membrane of the
germinal vesicle is in the act of commencing to atrophy, preparatory to
being extruded from the egg, while the contents of the germinal vesicle are
about to be absorbed.
In favour of the extrusion of the membrane rather than its absorption are
the following features:
(1) The thickness of its upper surface. (2) The extension of its edge over
the yolk. (3) Its position external to the yolk.
In favour of the view that the contents will be left behind and absorbed
when the membrane is pushed out, are the following features of my sections:
(1) The rupture of the membrane of the germinal vesicle on its lower
surface. (2) The position of the contents almost completely below the
membrane of the vesicle and surrounded by yolk.
In connection with this subject, Oellacher's valuable observations upon the
behaviour of the germinal vesicle in Osseous Fishes and in Birds at once
suggest themselves[72]. Oellacher sums up his results upon the behaviour of
the germinal vesicle in Osseous Fishes in the following way (p. 12):
"The germinal vesicle of the Trout's egg, at a period when
the egg is very nearly ripe, lies near the surface of the
germinal disc which is aggregated together in a hollow of
the yolk.... After this a hole appears in the membrane of
the germinal vesicle, which opens into the space between the
egg-membrane and the germinal disc. The hole widens more and
more, and the membrane frees itself little by little from
the contents of the germinal vesicle, which remain behind in
the form of a ball on the floor of the cavity formed in this
way. The cavity becomes flatter and flatter and the contents
are pushed up further and further from the germinal disc.
When the hollow, in which lie the contents of the original
germinal vesicle, completely vanishes, the covering membrane
becomes inverted ... and the membrane is spread out on the
convex surface of the germinal disc as a circular, investing
structure. It is clear that by the removal of the membrane
the contents of the germinal vesicle become lost."
Footnote 72: _Archiv für Micr. Anat._ Vol. VIII. p. 1.
These very definite statements of Oellacher tell strongly against my
interpretation of the appearance presented by the germinal vesicle of the
ripe Skate's egg. Oellacher's account is so precise, and his drawings so
fully bear out his interpretations, that it is very difficult to see where
any error can have crept in.
On the other hand, with the exception of those which Oellacher has made,
there cannot be said to be any satisfactory observations demonstrating the
extrusion of the germinal vesicle from the ovum. Oellacher has observed
this definitely for the Trout, but his observations upon the same point in
the Bird would quite as well bear the interpretation that the membrane
alone became pushed out, as that this occurred to the germinal vesicle,
contents and all.
While, then, there are on the one hand Oellacher's observations on a single
animal, hitherto unconfirmed, there are on the other very definite
observations tending to shew that the germinal vesicle has in many cases an
altogether different fate. Götte[73], not to mention other observers before
him, has in the case of Batrachian's eggs traced out with great precision
the gradual atrophy of the germinal vesicle, and its final absorption into
the matter of the ovum.
Footnote 73: _Entwicklungsgeschichte der Unke._
Götte distinguishes three stages in the degeneration of the germinal
vesicle of Bombinator's egg. In the first stage the germinal vesicle has
begun to travel up towards the surface of the egg. It retains nearly its
primitive condition, but its contents have become more opaque and have
partly withdrawn themselves from the thin membrane. The germinal spots are
still circular, but in some cases have increased in size. The most
important feature of this stage is the smaller size of the germinal vesicle
than that of the cavity of the yolk in which it lies, a condition which
appears to demonstrate the commencing atrophy of the vesicle.
In the next stage the cavity containing the germinal vesicle has vanished
without leaving a trace. The germinal vesicle itself has assumed a
lens-like form, and its borders are irregular and pressed in here and there
by yolk. Of the membrane of the germinal vesicle, and of the germinal
spots, only scanty remnants are to be seen, many of which lie in the
immediately adjoining yolk.
In the last stage no further trace of a distinct germinal vesicle is
present. In its place is a mass of very finely granular matter, which is
without a distinct border and graduates into the surrounding yolk and is to
be looked on as a remnant of the germinal vesicle.
This careful investigation of Götte proves beyond a doubt that in
Batrachians neither the membrane, nor the contents of the germinal vesicle,
are extruded from the egg.
In Mammalia, Van Beneden[74] finds that the germinal vesicle becomes
invisible, though he does not consider that it absolutely ceases to exist.
He has not traced the steps of the process with the same care as Götte, but
it is difficult to believe that an extrusion of the vesicle in the way
described by Oellacher would have escaped his notice.
Footnote 74: _Recherches sur la Composition et la Signification
de l'OEuf._
Passing from Vertebrates to Invertebrates, we find that almost every
careful investigator has observed the disappearance, apparent or otherwise,
of the germinal vesicle, but that very few have watched with care the steps
of the process.
The so-called Richtungskörper has been supposed to be the extruded remnant
of the germinal vesicle. This view has been especially adopted and
supported by Oellacher (_loc. cit._), and Flemming[75].
Footnote 75: "Studien in der Entwicklungsgeschichte der
Najaden," _Sitz. d. k. Akad. Wien_, Bd. LXXI. 1875.
The latter author regards the constant presence of this body, and the
facility with which it can be stained, as proofs of its connection with the
germinal vesicle, which has, however, according to his observations,
disappeared before the appearance of the Richtungskörper.
Kleinenberg[76], to whom we are indebted for the most precise observations
we possess on the disappearance of the germinal vesicle, gives the
following account of it, pp. 41 and 42.
"We left the germinal vesicle as a vesicle with a distinct
doubly contoured membrane, and equally distributed granular
contents, in which the germinal spot had appeared.... The
germinal vesicle reaches 0.06 mm. in diameter, and at the
same time its contents undergo a separation. The greater
part withdraws itself from the membrane and collects as a
dense mass around the germinal spot, while closely adjoining
the membrane there remains only a very thin but unbroken
lining of the plasmoid material. The intermediate space is
filled with a clear fluid, but the layer which lines the
membrane retains its connection with the mass around the
germinal vesicle by means of numerous fine threads which
traverse the space filled with fluid.... At about the time
when the formation of the pseudocells in the egg is
completed the germinal spot undergoes a retrogressive
metamorphosis, it loses its circular outline and it now
appears as if coagulated; then it breaks up into small
fragments, and I am fairly confident that these become
dissolved. The germinal vesicle ... becomes, on the egg
assuming a spherical form, drawn into an eccentric position
towards the pole of the egg directed outwards, where it lies
close to the surface and only covered by a very thin layer
of plasma. In this situation its degeneration now begins,
and ends in its complete disappearance. The granular
contents become more and more fluid; at the same time part
of them pass out through the membrane. This, which so far
was firmly stretched, next collapses to a somewhat egg-like
sac, whose wall is thickened and in places folded.
"The inner mass which up to this time has remained compact
now breaks up into separate highly refractive bodies, of
spherical or angular form and of very different sizes;
between them, here and there, are scattered drops of a fluid
fat.... I am very much inclined to regard the solid bodies
in question as fat or as that peculiar modification of
albuminoid bodies which we recognise as the certain
forerunner of the formation of fat in so many pathologically
altered tissues; and therefore to refer the disappearance of
the germinal vesicle to a fatty degeneration. On one
occasion I believe that I observed an opening in the
membrane at this stage; if this is a normal condition it
would be possible to believe that its solid contents passed
out and were taken up in the surrounding plasma. What
becomes of the membrane I am unable to say; in any case the
germinal vesicle has vanished to the very last trace before
impregnation occurs."
Footnote 76: _Hydra._ Leipzig, 1872.
Kleinenberg clearly finds that the germinal vesicle disappears completely
before the appearance of the Richtungskörper, in which he states a
pseudocell or yolk-sphere is usually found.
The connection between the Richtungskörper and the germinal vesicle is not
a result of strict observation, and there can be no question that the
evidence in the case of invertebrates tends to prove that the germinal
vesicle in no case disappears owing to its extrusion from the egg, but that
if part of it is extruded from the egg as Richtungskörper this occurs when
its constituents can no longer be distinguished from the remainder of the
yolk. This is clearly the case in Hydra, where, as stated above, one of the
pseudocells or yolk-spheres is usually found imbedded in the
Richtungskörper.
My observations on the Skate tend to shew that, in its case, the membrane
of the germinal vesicle is extruded from the egg, though they do not
certainly prove this. That conclusion is however supported by the
observations of Schenk[77]. He found in the impregnated, but not yet
segmented, germinal disc a cavity which, as he suggests, might well have
been occupied by the germinal vesicle. It is not unreasonable to suppose
that the membrane, being composed of formed matter and able only to take a
passive share in vital functions, could, without thereby influencing the
constitution of the ovum, be ejected.
Footnote 77: "Die Eier von Raja quadrimaculata," _Sitz. der k.
Akad. Wien_, Bd. LXVIII.
If we suppose, and this is not contradicted by observation, that the
Richtungskörper is either only the metamorphosed membrane of the germinal
vesicle with parts of the yolk, or part of the yolk alone, and assume that
in Oellacher's observations only the membrane and not the contents were
extruded from the egg, it would be possible to frame a consistent account
of the behaviour of the germinal vesicle throughout the animal kingdom,
which may be stated in the following way.
The germinal vesicle usually before, but sometimes immediately after
impregnation undergoes atrophy and its _contents_ become indistinguishable
from the remainder of the egg. In those cases in which its membrane is very
thick and resistent, _e.g._ Osseous and Elasmobranch Fishes, Birds, etc.,
this may be incapable of complete resorption, and be extruded bodily from
the egg. In the case of most ova, it is completely absorbed, though at a
subsequent period it may be extruded from the egg as the Richtungskörper.
In all cases the contents of the germinal vesicle remain in the ovum.
In some cases the germinal vesicle is stated to persist and to undergo
division during the process of segmentation; but the observations on this
point stand in need of confirmation.
My investigations shew that the germinal vesicle atrophies in the Skate
before impregnation, and in this respect accord with very many recent
observations. Of these the following may be mentioned.
(1) Oellacher (Bird, Osseous Fish). (2) Götte (Bombinator igneus). (3)
Kupffer (Ascidia canina). (4) Strasburger (Phallusia mamillata). (5)
Kleinenberg (Hydra). (6) Metschnikoff (Geryonia, Polyzenia leucostyla,
Epibulia aurantiaca, and other Hydrozoa).
This list is sufficient to shew that the disappearance of the germinal
vesicle before impregnation is very common, and I am unacquainted with any
observations tending to shew that its disappearance is due to impregnation.
In some cases, _e.g._ Asterocanthion[78], the germinal vesicle vanishes
after the spermatozoa have begun to surround the egg; but I do not know
that its disappearance in these cases has been shewn to be due to
impregnation. To do so it would be necessary to prove that in ripe eggs let
loose from the ovary, but not fertilized, the germinal vesicle did not
undergo the same changes as in the case of fertilized eggs; and this, as
far as I know, has not been done. After the disappearance of the germinal
vesicle, and before the first act of division, a fresh nucleus frequently
appears [--vide--Auerbach (Ascaris nigrovenosa), Fol (Geryonia), Kupffer
(Ascidia canina), Strasburger (Phallusia mamillata), Flemming (Anodon),
Götte (Bombinator igneus)], which is generally stated to vanish before the
appearance of the first furrow; but in some cases (Kupffer and Götte, and
as studied with especial care, Strasburger) it is stated to divide. Upon
the second nucleus, or upon its relation to the germinal vesicle, I have no
observations; but it appears to me of great importance to determine whether
this fresh nucleus arises absolutely de novo, or is formed out of the
matter of the germinal vesicle.
Footnote 78: Agassiz, _Embryology of the Star-Fish_.
The germinal vesicle is situated in a bed of finely divided yolk-particles.
These graduate insensibly into the coarser yolk-spherules around them,
though the band of passage between the coarse and the finer yolk-particles
is rather narrow. The mass of fine yolk-granules may be called the germinal
disc. It is not to be looked upon as diverging in any essential particular
from the remainder of the yolk, for the difference between the two is one
of degree only. It contains in fact a larger bulk of active protoplasm, as
compared with yolk-granules, than does the remainder of the ovum. The
existence of this agreement in kind has been already strongly insisted on
in my preliminary paper; and Schultz (_loc. cit._) has arrived at an
entirely similar conclusion, from his own independent observations.
One interesting feature about the germinal disc at this period is its size.
My observations upon it have been made with the eggs of the Skate (Raja)
alone; but I think that it is not probable that its size in the Skate is
greater than in Scyllium or Pristiurus. If its size is the same in all
these genera, then the germinal disc of the unimpregnated ovum is very much
greater than that portion of the ovum which undergoes segmentation, and
which is usually spoken of as the germinal disc in impregnated ova.
I have no further observation on the ripe ovarian ovum; and my next
observations concern an ovum in which two furrows have already appeared.