Cheiroptera may be directly derived from the primitive form by supposing
the villi of the discoidal placental area to have become more complex, so
as to form a deciduate discoidal placenta, while the yolk-sack still plays
a part, though physiologically an unimportant part, in rendering the
chorion vascular.
In the Carnivora, again, we have to start from the discoidal placenta, as
evinced by the fact that in the growth of the placenta the allantoic region
of the placenta is at first _discoidal_, and only becomes zonary at a later
stage. A zonary deciduate placenta indicates an increase both in area and
in complexity. The relative diminution of the breadth of the placental zone
in late foetal life in the zonary placenta of the Carnivora is probably due
to its being on the whole advantageous to secure the nutrition of the
foetus by insuring a more intimate relation between the foetal and maternal
parts, than by increasing their area of contact. The reason of this is not
obvious, but, as shewn below, there are other cases where it is clear that
a diminution in the area of the placenta has taken place, accompanied by an
increase in the complexity of its villi.
The second type of differentiation from the primitive form of placenta is
illustrated by the Lemuridæ, the Suidæ, and _Manis_. In all these cases the
area of the placental villi appears to have increased so as to cover nearly
the whole subzonal membrane, without the villi increasing to any great
extent in complexity. From the diffused placenta covering the whole surface
of the chorion, differentiations appear to have taken place in various
directions. The placenta of Man and Apes, from its mode of ontogeny, is
clearly derived from a diffused placenta (very probably similar to that of
Lemurs) by a concentration of the foetal villi, which are originally spread
over the whole chorion, to a disk-shaped area, and by an increase in their
arborescence. Thus the discoidal placenta of Man has no connexion with, and
ought not to be placed in, the same class as those of the Rodentia,
Cheiroptera, and Insectivora.
The polycotyledonary forms of placenta are due to similar concentrations of
the foetal villi of an originally diffused placenta.
In the Edentata we have a group with very varying types of placenta. Very
probably these may all be differentiations within the group itself from a
diffused placenta such as that found in _Manis_. The zonary placenta of
_Orycteropus_ is capable of being easily derived from that of _Manis_ by
the disappearance of the foetal villi at the two poles of the ovum. The
small size of the umbilical vesicle in _Orycteropus_ indicates that its
discoidal placenta is not, like that of the Carnivora, directly derived
from a type with both allantoic and umbilical vascularization of the
chorion. The discoidal and dome-shaped placentæ of the Armadillos,
_Myrmecophaga_, and the Sloths may easily have been formed from a diffused
placenta, just as the discoidal placenta of the Simiidæ and Hominidæ
appears to have been formed from a diffused placenta like that of the
Lemuridæ.
The presence of zonary placenta in _Hyrax_ and _Elephas_ does not
necessarily afford any proof of affinity of these types with the Carnivora.
A zonary placenta may be quite as easily derived from a diffused placenta
as from a discoidal placenta; and the presence of two villous patches at
the poles of the chorion in _Elephas_ very probably indicates that its
placenta has been evolved from a diffused placenta.
Although it would not be wise to attempt to found a classification upon the
placental characters alone, it may be worth while to make a few suggestions
as to the affinities of the orders of Mammalia indicated by the structure
of the placenta. We clearly, of course, have to start with forms which
could not be grouped with any of the existing orders, but which might be
called the Protoplacentalia. They probably had the primitive type of
placenta described above: the nearest living representatives of the group
are the Rodentia, Insectivora, and Cheiroptera. Before, however, these
three groups had become distinctly differentiated, there must have branched
off from the primitive stock the ancestors of the Lemuridæ, the Ungulata,
and the Edentata.
It is obvious on general anatomical grounds that the Monkeys and Man are to
be derived from a primitive Lemurian type; and with this conclusion the
form of the placenta completely tallies. The primitive Edentata and
Ungulata had no doubt a diffused placenta which was probably not very
different from that of the primitive Lemurs; but how far these groups arose
quite independently from the primitive stock, or whether they may have had
a nearer common ancestor, cannot be decided from the structure of the
placenta. The Carnivora were certainly an offshoot from the primitive
placental type which was quite independent of the three groups just
mentioned; but the character of the placenta of the Carnivora does not
indicate at what stage in the evolution of the placental Mammalia a
primitive type of Carnivora was first differentiated.
No important light is thrown by the placenta on the affinities of the
Proboscidea, the Cetacea, or the Sirenia; but the character of the placenta
in the latter group favours the view of their being related to the
Ungulata.
XXII. ON THE STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS[498].
By F. M. BALFOUR and W. N. PARKER.
Footnote 498: From the _Philosophical Transactions of the
Royal Society_, 1882.
(With Plates 34-42.)
TABLE OF CONTENTS.
PAGE
INTRODUCTION 739
GENERAL DEVELOPMENT 740
BRAIN--
Adult brain 759
Development of the brain 764
Comparison of the larval and adult brain of
_Lepidosteus_, together with some observations on
the systematic value of the characters of the
Ganoid brain 767
SENSE ORGANS--
Olfactory organ 771
Anatomy of the eye _ib._
Development of the eye 772
SUCTORIAL DISC 774
MUSCULAR SYSTEM 775
SKELETON--
Vertebral column and ribs of the adult 776
Development of the vertebral column and ribs. 778
Comparison of the vertebral column of _Lepidosteus_
with that of other forms 792
The ribs of Fishes 793
The skeleton of the ventral lobe of the tail fin, and
its bearing on the nature of the tail fin of the
various types of Pisces 801
EXCRETORY AND GENERATIVE ORGANS--
Anatomy of the excretory and generative organs of the
female 810
Anatomy of the excretory and generative organs of the
male 813
Development of the excretory and generative organs 815
Theoretical considerations 822
THE ALIMENTARY CANAL AND ITS APPENDAGES--
Topographical anatomy of the alimentary canal 828
Development of the alimentary canal and its appendages 831
THE GILL ON THE HYOID ARCH 835
THE SYSTEMATIC POSITION OF LEPIDOSTEUS 836
LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF
LEPIDOSTEUS 840
LIST OF REFERENCE LETTERS 841
EXPLANATION OF PLATES 842
INTRODUCTION.
The following paper is the outcome of the very valuable gift of a series of
embryos and larvæ of _Lepidosteus_ by Professor Alex. Agassiz, to whom we
take this opportunity of expressing our most sincere thanks. The skull of
these embryos and larvæ has been studied by Professor Parker, and forms the
subject of a memoir already presented to the Royal Society.
Considering that _Lepidosteus_ is one of the most interesting of existing
Ganoids, and that it is very closely related to species of Ganoids which
flourished during the Triassic period, we naturally felt keenly anxious to
make the most of the opportunity of working at its development offered to
us by Professor Agassiz' gift. Professor Agassiz, moreover, most kindly
furnished us with four examples of the adult Fish, which have enabled us to
make this paper a study of the adult anatomy as well as of the development.
The first part of our paper is devoted to the segmentation, formation of
the germinal layers, and general development of the embryo and larva. The
next part consists of a series of sections on the organs, in which both
their structure in the adult and their development are dealt with. This
part is not, however, in any sense a monograph, and where already known,
the anatomy is described with the greatest possible brevity. In this part
of the paper considerable space is devoted to a comparison of the organs of
_Lepidosteus_ with those of other Fishes, and to a statement of the
conclusions which follow from such comparison.
The last part of the paper deals with the systematic position of
_Lepidosteus_ and of the Ganoids generally.
GENERAL DEVELOPMENT.
The spawning of _Lepidosteus_ takes place in the neighbourhood of New York
about May 20th. Agassiz (No. 1)[499] gives an account of the process from
Mr S. W. Garman's notes, which we venture to quote in full.
Footnote 499: The numbers refer to the list of memoirs of the
anatomy and development given at the end of this memoir.
"Black Lake is well stocked with Bill-fish. When they appear, they are
said to come in countless numbers. This is only for a few days in the
spring, in the spawning season, between the 15th of May and the 8th of
June. During the balance of the season they are seldom seen. They
remain in the deeper parts of the lake, away from the shore, and,
probably, are more or less nocturnal in habits. Out of season, an
occasional one is caught on a hook baited with a minnow. Commencing
with the 20th of April, until the 14th of May we were unable to find
the Fish, or to find persons who had seen them during this time. Then
a fisherman reported having seen one rise to the surface. Later,
others were seen. On the afternoon of the 18th, a few were found on
the _points_, depositing the spawn. The temperature at the time was
68° to 69° on the shoals, while out in the lake the mercury stood at
62° to 63°. The _points_ on which the eggs were laid were of naked
granite, which had been broken by the frost and heat into angular
blocks of 3 to 8 inches in diameter. The blocks were tumbled upon each
other like loose heaps of brick-bats, and upon and between them the
eggs were dropped. The _points_ are the extremities of small capes
that make out into the lake. The eggs were laid in water varying in
depth from 2 to 14 inches. At the time of approaching the shoals, the
Fish might be seen to rise quite often to the surface to take air.
This they did by thrusting the bill out of the water as far as the
corners of the mouth, which was then opened widely and closed with a
snap. After taking the air, they seemed more able to remain at the
surface. Out in the lake they are very timid, but once buried upon the
shoals they become quite reckless as to what is going on about them. A
few moments after being driven off, one or more of the males would
return as if scouting. If frightened, he would retire for some time;
then another scout would appear. If all promised well, the females,
with the attendant males, would come back. Each female was accompanied
by from one to four males. Most often, a male rested against each
side, with their bills reaching up toward the back of her head.
Closely crowded together, the little party would pass back and forth
over the rocky bed they had selected, sometimes passing the same spot
half-a-dozen times without dropping an egg, then suddenly would
indulge in an orgasm; and, lashing and plashing the water in all
directions with their convulsive movements, would scatter at the same
instant the eggs and the sperm. This ended, another season of moving
slowly back and forth was observed, to be in turn followed by another
of excitement. The eggs were excessively sticky. To whatever they
happened to touch, they stuck, and so tenaciously that it was next to
impossible to release them without tearing away a portion of their
envelopes. It is doubtful whether the eggs would hatch if removed. As
far as could be seen at the time, upon or under the rocks to which the
eggs were fastened there was an utter absence of anything that might
serve as food for the young Fishes.
"Other Fishes, Bull-heads, &c., are said to follow the Bill-fish to
eat the spawn. It may be so. It was not verified. Certainly the points
under observations were unmolested. During the afternoon of the 18th
of May a few eggs were scattered on several of the beds. On the 19th
there were more. With the spear and the snare, several dozens of both
sexes of the Fish were taken. Taking one out did not seem greatly to
startle the others. They returned very soon. The males are much
smaller than the average size of the females; and, judging from those
taken, would seem to have as adults greater uniformity in size. The
largest taken was a female, of 4 feet 1-1/2 inch in length. Others of
2 feet 6 inches contained ripe ova. With the 19th of May all
disappeared, and for a time--the weather being meanwhile cold and
stormy--there were no signs of their continued existence to be met
with. Nearly two weeks later, on the 31st of May, as stated by Mr
Henry J. Perry, they again came up, not in small detachments on
scattered points as before, but in multitudes, on every shoal at all
according with their ideas of spawning beds. They remained but two
days. During the summer it happens now and then that one is seen to
come up for his mouthful of air; beyond this there will be nothing to
suggest the ravenous masses hidden by the darkness of the waters."
_Egg membranes._--The ova of _Lepidosteus_ are spherical bodies of about 3
millims. in diameter. They have a double investment consisting of (1) an
outer covering formed of elongated, highly refractive bodies, somewhat
pyriform at their outer ends (Plate 34, fig. 17, _f.e._), which are
probably metamorphosed follicular cells[500], and (2) of an inner membrane,
divided into two zones, viz.: an outer and thicker zone, which is radially
striated, and constitutes the _zona radiata (z.r.)_, and an inner and
narrow homogeneous zone (_z.r´._).
Footnote 500: We have examined the structure of the ovarian
ova in order to throw light on the nature of these peculiar
pyriform bodies. Unfortunately, the ovaries of our adult
examples of _Lepidosteus_ were so badly preserved, that we
could not ascertain anything on this subject. The ripe ova in
the ovary have an investment of pyriform bodies similar to
those of the just laid ova. With reference to the structure of
the ovarian ova we may state that the germinal vesicles are
provided with numerous nucleoli arranged in close proximity
with the membrane of the vesicle.
_Segmentation._--We have observed several stages in the segmentation, which
shew that it is complete, but that it approaches the meroblastic type more
nearly than in the case of any other known holoblastic ovum.
Our earliest stage shewed a vertical furrow at the upper or animal pole,
extending through about one-fifth of the circumference (Plate 34, fig. 1),
and in a slightly later stage we found a second similar furrow at right
angles to the first (Plate 34, fig. 2). We have not been fortunate enough
to observe the next phases of the segmentation, but on the second day after
impregnation (Plate 34, fig. 3), the animal pole is completely divided into
small segments, which form a disc, homologous to the blastoderm of
meroblastic ova; while the vegetative pole, which subsequently forms a
large yolk-sack, is divided by a few vertical furrows, four of which nearly
meet at the pole opposite the blastoderm (Plate 34, fig. 4). The majority
of the vertical furrows extend only a short way from the edge of the small
spheres, and are partially intercepted by imperfect equatorial furrows.
_Development of the embryo._--We have not been able to work out the stages
immediately following the segmentation, owing to want of material; and in
the next stage satisfactorily observed, on the third day after
impregnation, the body of the embryo is distinctly differentiated. The
lower pole of the ovum is then formed of a mass in which no traces of the
previous segments or segmentation furrows could any longer be detected.
Some of the dates of the specimens sent to us appear to have been
transposed; so that our statements as to ages must only be taken as
_approximately_ correct.
_Third day after impregnation._--In this stage the embryo is about 3.5
millims. in length, and has a somewhat dumb-bell shaped outline (Plate 34,
fig. 5). It consists of (1) an outer area (_p.z_) with some resemblance to
the area pellucida of the Avian embryo, forming the parietal part of the
body; and (2) a central portion consisting of the vertebral and medullary
plates and the axial portions of the embryo. In hardened specimens the
peripheral part forms a shallow depression surrounding the central part of
the embryo.
The central part constitutes a somewhat prominent ridge, the axial part of
it being the medullary plate. Along the anterior half of this part a dark
line could be observed in all our specimens, which we at first imagined to
be caused by a shallow groove. We have, however, failed to find in our
sections a groove in this situation except in a single instance (Plate 35,
fig. 20, _x_), and are inclined to attribute the appearance above-mentioned
to the presence of somewhat irregular ridges of the outer layer of the
epiblast, which have probably been artificially produced in the process of
hardening.
The anterior end of the central part is slightly dilated to form the brain
(_b_); and there is present a pair of lateral swellings near the anterior
end of the brain which we believe to be the commencing optic vesicles. We
could not trace any other clear indications of the differentiation of the
brain into distinct lobes.
At the hinder end of the central part of the embryo a very distinct
dilatation may also be observed, which is probably homologous with the tail
swelling of Teleostei. Its structure is more particularly dealt with in the
description of our sections of this stage.
After the removal of the egg-membranes described above we find that there
remains a delicate membrane closely attached, to the epiblast. This
membrane can be isolated in distinct portions, and appears to be too
definite to be regarded as an artificial product.
We have been able to prepare several more or less complete series of
sections of embryos of this stage (Plate 35, figs. 18-22). These sections
present as a whole a most striking resemblance to those of Teleostean
embryos at a corresponding stage of development.
Three germinal layers are already fully established. The epiblast (_ep._)
is formed of the same parts as in Teleostei, viz.:--of an outer epidermic
and an inner nervous or mucous stratum. In the parietal region of the
embryo these strata are each formed of a single row of cells only. The
cells of both strata are somewhat flattened, but those of the epidermic
stratum are decidedly the more flattened of the two.
Along the axial line there is placed, as we have stated above, the
medullary plate. The epidermic stratum passes over this plate without
undergoing any change of character, and the plate is _entirely constituted
of the nervous stratum of the epidermis_.
The medullary plate has, roughly speaking, the form of a solid keel,
projecting inwards towards the yolk. There is no trace, at this stage at
any rate, of a medullary groove; and as, we shall afterwards shew, the
central canal of the cerebro-spinal cord is formed in the middle of the
solid keel. The shape of this keel varies according to the region of the
body. In the head (Plate 35, fig. 18, _m.c._), it is very prominent, and
forming, as it does, the major part of the axial tissue of the body,
impresses its own shape on the other parts of the head and gives rise to a
marked ridge on the surface of the head directed towards the yolk. In the
trunk (Plate 35, figs. 19, 20) the keel is much less prominent, but still
projects sufficiently to give a convex form to the surface of the body
turned towards the yolk.
In the head, and also near the hind end of the trunk, the nervous layer of
the epiblast continuous with the keel on each side is considerably thicker
than the lateral parts of the layer. The thickening of the nervous layer in
the head gives rise to what has been called by Götte[501] "the special
sense plate," owing to its being subsequently concerned in the formation of
parts of the organs of special sense. We cannot agree with Götte in
regarding it as part of the brain.
Footnote 501: "Ueb. d. Entwick. d. Central Nerven Systems d.
Teleostier," _Archiv für mikr. Anat._ Vol. XV. 1878.
In the keel itself two parts may be distinguished, viz.: a superficial
part, best marked in the region of the brain, formed of more or less
irregularly arranged polygonal cells, and a deeper part of horizontally
placed flatter cells. The upper part is mainly concerned in the formation
of the cranial nerves, and of the dorsal roots of the spinal nerves.
The mesoblast (_ms._) in the trunk consists of a pair of independent plates
which are continued forwards into the head, and in the prechordal region of
the latter, unite below the medullary keel.
The mesoblastic plates of the trunk are imperfectly divided into vertebral
and lateral regions. Neither longitudinal sections nor surface views shew
at this stage any trace of a division of the mesoblast into somites. The
mesoblast cells are polygonal, and no indication is as yet present of a
division into splanchnic and somatic layers.
The notochord (_nc._) is well established, so that its origin could not be
made out. It is, however, much more sharply separated from the mesoblastic
plates than from the hypoblast, though the ventral and inner corners of the
mesoblastic plates which run in underneath it on either side, are often
imperfectly separated from it. It is formed of polygonal cells, of which
between 40 and 50 may as a rule be seen in a single section. No sheath is
present around it. It has the usual extension in front.
The hypoblast (_hy._) has the form of a membrane, composed of a single row
of oval cells, bounding the embryo on the side adjoining the yolk.
In the region of the caudal swelling the relations of the germinal layers
undergo some changes. This region may, from the analogy of other
Vertebrates, be assumed to constitute the lip of the blastopore. We find
accordingly that the layers become more or less fused. In the anterior part
of the tail swelling, the boundary between the notochord and hypoblast
becomes indistinct. A short way behind this point (Plate 35, fig. 21), the
notochord unites with the medullary keel, and a neurenteric cord,
homologous with the neurenteric canal of other Ichthyopsida, is thus
established. In the same region the boundary between the lateral plates of
mesoblast and the notochord, and further back (Plate 35, fig. 22), that
between the mesoblast and the medullary keel, becomes obliterated.
_Fifth day after impregnation._--Between the stage last described and the
next stage of which we have specimens, a considerable progress has been
made. The embryo (Plate 34, figs. 6 and 7) has grown markedly in length and
embraces more than half the circumference of the ovum. Its general
appearance is, however, much the same as in the earlier stage, but in the
cephalic region the medullary plate is divided by constrictions into three
distinct lobes, constituting the regions of the fore-brain, the mid-brain,
and the hind-brain. The fore-brain (Plate 34, fig. 6, _f.b._) is
considerably the largest of the three lobes, and a pair of lateral
projections forming the optic vesicles are decidedly more conspicuous than
in the previous stage. The mid-brain (_m.b._) is the smallest of the three
lobes, while the hind-brain (_h.b._) is decidedly longer, and passes
insensibly into the spinal cord behind.
The medullary keel, though retaining to a great extent the shape it had in
the last stage, is no longer completely solid. Throughout the whole region
of the brain and in the anterior part of the trunk (Plate 35, figs. 23, 24,
25) a slit-like lumen has become formed. We are inclined to hold that this
is due to the appearance of a space between the cells, and not, as supposed
by Oellacher for Teleostei, to an actual absorption of cells, though we
must admit that our sections are hardly sufficiently well preserved to be
conclusive in settling this point. Various stages in its growth may be
observed in different regions of the cerebro-spinal cord. When first
formed, it is a very imperfectly defined cavity, and a few cells may be
seen passing right across from one side of it to the other. It gradually
becomes more definite, and its wall then acquires a regular outline.
The optic vesicles are now to be seen in section (Plate 35, fig. 23, _op._)
as flattish outgrowths of the wall of the fore-brain, into which the lumen
of the third ventricle is prolonged for a short distance.
The brain has become to some extent separate from the superjacent epiblast,
but the exact mode in which this is effected is not clear to us. In some
sections it appears that the separation takes place in such a way that the
nervous keel is only covered above by the epidermic layer of the epiblast,
and that the nervous layer, subsequently interposed between the two, grows
in from the two sides. Such a section is represented in Plate 35, fig. 24.
Other sections again favour the view that in the isolation of the nervous
keel, a superficial layer of it remains attached to the nervous layer of
the epidermis at the two sides, and so, from the first, forms a continuous
layer between the nervous keel and the epidermic layer of the epiblast
(Plate 35, fig. 25). In the absence of a better series of sections we do
not feel able to determine this point. The posterior part of the nervous
keel retains the characters of the previous stage.
At the sides of the hind-brain very distinct commencements of the auditory
vesicles are apparent. They form shallow pits (Plate 35, fig. 24, _au._) of
the thickened part of the nervous layer adjoining the brain in this region.
Each pit is covered over by the epidermic layer above, which has no share
in its formation.
In many parts of the lateral regions of the body the nervous layer of the
epidermis is more than one cell deep.
The mesoblastic plates are now divided in the anterior part of the trunk
into a somatic and a splanchnic layer (Plate 35, fig. 25, _so._, _sp._),
though no distinct cavity is as yet present between these two layers. Their
vertebral extremities are somewhat wedge-shaped in section, the base of the
wedge being placed at the sides of the medullary keel. The wedge-shaped
portions are formed of a superficial layer of palisade-like cells and an
inner kernel of polygonal cells. The superficial layer on the dorsal side
is continuous with the somatic mesoblast, while the remainder pertains to
the splanchnic layer.
The diameter of the notochord has diminished, and the cells have assumed a
flattened form, the protoplasm being confined to an axial region. In
consequence of this, the peripheral layer appears clear in transverse
sections. A delicate cuticular sheath is formed around it. This sheath is
probably the commencement of the permanent sheath of later stages, but at
this stage it cannot be distinguished in structure from a delicate cuticle
which surrounds the greater part of the medullary cord.
The hypoblast has undergone no changes of importance.
The layers at the posterior end of the embryo retain the characters of the
last stage.
_Sixth day after impregnation._--At this stage (Plate 34, fig. 8) the
embryo is considerably more advanced than at the last stage. The trunk has
decidedly increased in length, and the head forms a relatively smaller
portion of the whole. The regions of the brain are more distinct. The optic
vesicles (_op._) have grown outwards so as to nearly reach the edges of the
area which forms the parietal part of the body. The fore-brain projects
slightly in front, and the mid-brain is seen as a distinct rounded
prominence. Behind the latter is placed the hind-brain, which passes
insensibly into the spinal cord. On either side of the mid- and hind-brain
a small region is slightly marked off from the rest of the parietal part,
and on this are seen two more or less transversely directed streaks, which,
by comparison with the Sturgeon[502], we are inclined to regard as the two
first visceral clefts (_br.c._). We have, however, failed to make them out
in sections, and owing to the insufficiency of our material, we have not
even studied them in surface views as completely as we could have wished.
Footnote 502: Salensky, "Recherches s. le Développement du
Sterlet." _Archives de Biol._ Vol. II. 1881, pl. XVII.
fig. 27.
The body is now laterally compressed, and more decidedly raised from the
yolk than in the previous stages. In the lateral regions of the trunk the
two segmental or archinephric ducts (_sg._) are visible in surface views:
the front end of each is placed at the level of the hinder border of the
head, and is marked by a flexure inwards towards the middle line. The
remainder of each duct is straight, and extends backwards for about half
the length of the embryo. The tail has much the same appearance as in the
last stage.
The vertebral regions of the mesoblastic plates are now segmented for the
greater part of the length of the trunk, and the somites of which they are
composed (Plate 36, fig. 30, _pr._) are very conspicuous in surface views.
Our sections of this stage are not so complete as could be desired: they
shew, however, several points of interest.
The central canal of the nervous system is large, with well-defined walls,
and in hardened specimens is filled with a coagulum. It extends nearly to
the region of the tail.
The optic vesicles, which are so conspicuous in surface views, appear in
section (Plate 35, fig. 26, _op._) as knob-like outgrowths of the
fore-brain, and very closely resemble the figures given by Oellacher of
these vesicles in Teleostei[503].
Footnote 503: "Beiträge zur Entwick. d. Knochenfische," _Zeit.
f. wiss. Zool._ Vol. XXIII. 1873, taf. III. fig. IX. 2.
From the analogy of the previous stage, we are inclined to think that they
have a lumen continuous with that of the fore-brain. In our only section
through them, however, they are solid, but this is probably due to the
section merely passing through them to one side.
The auditory pits (Plate 35, fig. 27, _au._) are now well marked, and have
the form of somewhat elongated grooves, the walls of which are formed of a
single layer of columnar cells belonging to the nervous layer of the
epidermis, and extending inwards so as nearly to touch the brain.
In an earlier stage it was pointed out that the dorsal part of the
medullary keel was different in its structure from the remainder, and that
it was destined to give rise to the nerves. The process of differentiation
is now to a great extent completed, and may best be seen in the auditory
region (Plate 35, fig. 27, VIII.). In this region there was present during
the last stage a great rhomboidal mass of cells at the dorsal region of the
brain (Plate 35, fig. 24, VIII.). In the present stage, this, which is the
rudiment of the seventh and auditory nerves, is seen growing down on each
side from the roof of the hind-brain, between the brain and the auditory
involution, and abutting against the wall of the latter.
Rudiments of the spinal nerves are also seen at intervals as projections
from the dorsal angles of the spinal cord (Plate 36, fig. 29, _sp.n._).
They extend only for a short distance outwards, gradually tapering off to a
point, and situated between the epiblast and the dorsal angles of the
mesoblastic somites.
The process of formation of the cranial nerves and dorsal roots of the
spinal nerves is, it will be seen, essentially the same as that already
known in the case of Elasmobranchii, Aves, &c. The nerves arise as
outgrowths of a special crest of cells, the _neural crest_ of Marshall,
which is placed along the dorsal angle of the cord. The peculiar position
of the dorsal roots of the spinal nerves is also very similar to what has
been met with in the early stages of these structures by Marshall in
Birds[504], and by one of us in Elasmobranchii[505].
Footnote 504: _Journal of Anat. and Physiol._ Vol. XI. p. 491,
plates XX. and XXI.
Footnote 505: "Elasmobranch Fishes," p. 156, plates 10 and 13.
[This edition, p. 378, pl. 11, 14.]
In the parietal region a cavity has now appeared in part of the trunk
between the splanchnic and somatic layers of the mesoblast (Plate 36,
fig. 29, _b.c._), the somatic layer (_so._) consisting of a single row of
columnar cells on the dorsal side, while the remainder of each somite is
formed of the splanchnic layer (_sp._). In many of the sections the somatic
layer is separated by a considerable interval from the epiblast.
We have been able to some extent to follow the development of the segmental
duct. The imperfect preservation of our specimens has, as in other
instances, rendered the study of the point somewhat difficult, but we
believe that the figure representing the development of the duct some way
behind its front end (Plate 36, fig. 29) is an accurate representation of
what may be seen in a good many of our sections.
It appears from these sections that the duct (Plate 36, fig. 29, _sg._) is
developed as a hollow ridge-like outgrowth of the somatic layer of
mesoblast, directed towards the epiblast, in which it causes a slight
bulging. The cavity of the ridge freely communicates with the body-cavity.
The anterior part of this ridge appears to be formed first. Very soon, in
fact, in an older embryo belonging to this stage, the greater part of the
groove becomes segmented off as a duct lying between the epiblast and
somatic mesoblast (Plate 36, fig. 28, _sg._), while the front end still
remains, as we believe, in communication with the body-cavity by an
anterior pore.
This mode of development corresponds in every particular with that observed
in Teleostei by Rosenberg and Oellacher.
The structure of the notochord (_nc._) at this stage is very similar to
that observed by one of us in Elasmobranchii[506]. The cord is formed of
transversely arranged flattened cells, the outer parts of which are
vacuolated, while the inner parts are granular, and contain the nuclei.
This structure gives rise to the appearance in transverse sections of an
axial darker area and a peripheral lighter portion.
Footnote 506: "Elasmobranch Fishes," p. 136, plate 11,
fig. 10. [This edition, p. 354, pl. 12.]
The hypoblast retains for the most part its earlier constitution, but
underneath the notochord, in the trunk, it is somewhat thickened, and the
cells at the two sides spread in to some extent under the thickened portion
(Plate 36, fig. 29, _s.nc._). This thickening, as is shewn in transverse
sections at the stage when the segmental duct becomes separated from the
somatic mesoblast (Plate 36, fig. 28, _s.nc._), is the commencement of the
subnotochordal rod.
The tail end of the embryo still retains its earlier characters.
_Seventh day after impregnation._--Our series of specimens of this stage is
very imperfect, and we are only able to call attention to the development
of a certain number of organs.
Our sections clearly establish the fact that the optic vesicles are now
hollow processes of the fore-brain. Their outer ends are dilated, and are
in contact with the external skin. The formation of the optic cup has not,
however, commenced. The nervous layer of the skin adjoining the outer wall
of the optic cup is very slightly thickened, constituting the earliest
rudiment of the lens.
In one of our embryos of this day the developing auditory vesicle still has
the form of a pit, but in the other it is a closed vesicle, already
constricted off from the nervous layer of the epidermis.
With reference to the development of the excretory duct we cannot add much
to what we have already stated in describing the last stage.
The duct is considerably dilated anteriorly (Plate 36, fig. 31, _sg._); but
our sections throw no light on the nature of the abdominal pore. The
posterior part of the duct has still the form of a hollow ridge united with
somatic mesoblast (Plate 36, fig. 32, _sg._).
During this stage, the embryo becomes to a small extent folded off from the
yolk-sack both in front and behind, and in the course of this process the
anterior and posterior extremities of the alimentary tract become
definitely established.
We have not got as clear a view of the process of formation of these two
sections of the alimentary tract as we could desire, but our observations
appear to shew that the process is in many respects similar to that which
takes place in the formation of the anterior part of the alimentary tract
in Elasmobranchii[507]. One of us has shewn that in Elasmobranchii the
ventral wall of the throat is formed _not_ by a process of folding in of
the hypoblastic sheet as in Birds, but by a growth of the ventral face of
the hypoblastic sheet on each side of and at some little distance from the
middle line. Each growth is directed inwards, and the two eventually meet
and unite, thus forming a complete ventral wall for the gut. Exactly the
same process would seem to take place in _Lepidosteus_, and after the lumen
of the gut is in this way established, a process of mesoblast on each side
also makes its appearance, forming a mesoblastic investment on the ventral
side of the alimentary tract. Some time after the alimentary tract has been
thus formed, the epiblast becomes folded in, in exactly the same manner as
in the Chick, the embryo becoming thereby partially constricted off from
the yolk (Plate 36, figs. 33, 34).
Footnote 507: F. M. Balfour, "Monograph on the Development of
Elasmobranch Fishes," p. 87, plate 9, fig. 2. [This edition, p.
303, pl. 10.]
The form of the lumen of the alimentary tract differs somewhat in front and
behind. In front, the hypoblastic sheet remains perfectly flat during the
formation of the throat, and thus the lumen of the latter has merely the
form of a slit. The lumen of the posterior end of the alimentary tract is,
however, narrower and deeper (Plate 36, figs. 33, 34, _al._). Both in front
and behind, the lateral parts of the hypoblastic sheet become separated
from the true alimentary tract as soon as the lumen of the latter is
established.
It is quite possible that at the extreme posterior end of the embryo a
modification of the above process may take place, for in this region the
hypoblast appears to us to have the form of a solid cord.
We could detect no true neurenteric canal, although a more or less complete
fusion of the germinal layers at the tail end of the embryo may still be
traced.
During this stage the protoplasm of the notochordal cells, which in the
last stage formed a kind of axial rod in the centre of the notochord,
begins to spread outwards toward the sheath of the notochord.
_Eighth day after impregnation._--The external form of the embryo (Plate
34, fig. 9) shews a great advance upon the stage last figured. Both head
and body are much more compressed laterally and raised from the yolk, and
the head end is folded off for some distance. The optic vesicles are much
less prominent externally. A commencing opercular fold is distinctly seen.
Our figure of this stage is not, however, so satisfactory as we could wish.
A thickening of the nervous layer of the external epiblast which will form
the lens (Plate 36, fig. 35, _l._) is more marked than in the last stage,
and presses against the slightly concave exterior wall of the optic vesicle
(_op._). The latter has now a large cavity, and its stalk is considerably
narrowed.
The auditory vesicles (Plate 36, fig. 36, _au._) are closed, appearing as
hollow sacks one on each side of the brain, and are no longer attached to
the epiblast.
The anterior opening of the segmental duct can be plainly seen close behind
the head. The lumen of the duct is considerably larger.
The two vertebral portions of the mesoblast are now separated by a
considerable space from the epiblast on one side and from the notochord on
the other, and the cells composing them have become considerably elongated
from side to side (Plate 36, fig. 37, _ms_).
In some sections the aorta can be seen (Plate 36, fig. 37, _ao_) lying
close under the subnotochordal rod, between it and the hypoblast, and on
either side of it a slightly larger cardinal vein (_cd.v._).
The protoplasm of the notochord has now again retreated towards the centre,
shewing a clear space all round. This is most marked in the region of the
trunk (Plate 36, fig. 37). The subnotochordal rod (_s.nc._) lies close
under it.
A completely closed fore-gut, lined by thickened hypoblast, extends about
as far back as the auditory sacks (Plate 36, figs. 35 and 36, _al._). In
the trunk the hypoblast, which will form the walls of the alimentary tract,
is separated from the notochord by a considerable interval.
_Ninth day after impregnation: External characters._--Very considerable
changes have taken place in the external characters of the embryo. It is
about 8 millims. in length, and has assumed a completely piscine form. The
tail especially has grown in length, and is greatly flattened from side to
side: it is wholly detached from the yolk, and bends round towards the
head, usually with its left side in contact with the yolk. It is provided
with well-developed dorsal and ventral fin-folds, which meet each other
round the end of the tail, the tail fin so formed being nearly symmetrical.
The head is not nearly so much folded off from the yolk as the tail. At its
front end is placed a disc with numerous papillæ, of which we shall say
more hereafter. This disc is somewhat bifid, and is marked in the centre by
a deep depression.
Dorsal to it, on the top of the head, are two widely separated nasal pits.
On the surface of the yolk, in front of the head, is to be seen the heart,
just as in Sturgeon embryos. Immediately below the suctorial disc is a
slit-like space, forming the mouth. It is bounded below by the two
mandibular arches, which meet ventrally in the median line. A shallow but
well-marked depression on each side of the head indicates the posterior
boundary of the mandibular arch. Behind this is placed the very conspicuous
hyoid arch with its rudimentary opercular flap; and in the depression,
partly covered over by the latter, may be seen a ridge, the external
indication of the first branchial arch.
_Eleventh day after impregnation: External characters._--The embryo (Plate
34, fig. 10) is now about 10 millims. in length, and in several features
exhibits an advance upon the embryo of the previous stage.
The tail fin is now obviously not quite symmetrical, and the dorsal
fin-fold is continued for nearly the whole length of the trunk. The
suctorial disc (Plate 34, fig. 11, _s.d._) is much more prominent, and the
papillæ (about 30 in number) covering it are more conspicuous from the
surface. It is not obviously composed of two symmetrical halves. The
opercular flap is larger, and the branchial arches behind it (two of which
may be made out without dissection) are more prominent.
The anterior pair of limbs is now visible in the form of two _longitudinal_
folds projecting in a vertical direction from the surface of the yolk-sack
at the sides of the body.
The stages subsequent to hatching have been investigated with reference to
the external features and to the habits by Agassiz, and we shall enrich our
own account by copious quotations from his memoir.
He states that the first batch were hatched on the eighth[508] day after
being laid. "The young Fish possessed a gigantic yolk-bag, and the
posterior part of the body presented nothing specially different from the
general appearance of a Teleostean embryo, with the exception of the great
size of the chorda. The anterior part, however, was most remarkable; and at
first, on seeing the head of this young _Lepidosteus_, with its huge
mouth-cavity extending nearly to the gill-opening, and surmounted by a
hoof-shaped depression edged with a row of protuberances acting as suckers,
I could not help comparing this remarkable structure, so utterly unlike
anything in Fishes or Ganoids, to the Cyclostomes, with which it has a
striking analogy. This organ is also used by _Lepidosteus_ as a sucker, and
the moment the young Fish is hatched he attaches himself to the sides of
the disc, and there remains hanging immovable; so firmly attached, indeed,
that it requires considerable commotion in the water to make him loose his
hold. Aërating the water by pouring it from a height did not always produce
sufficient disturbance to loosen the young Fishes. The eye, in this stage,
is rather less advanced than in corresponding stages in bony Fishes; the
brain is also comparatively smaller, the otolith ellipsoidal, placed
obliquely in the rear above the gill-opening.... Usually the gill-cover is
pressed closely against the sides of the body, but in breathing an opening
is seen through which water is constantly passing, a strong current being
made by the rapid movement of the pectorals, against the base of which the
extremity of the gill-cover is closely pressed. The large yolk-bag is
opaque, of a bluish-gray colour. The body of the young _Lepidosteus_ is
quite colourless and transparent. The embryonic fin is narrow, the dorsal
part commencing above the posterior end of the yolk-bag; the tail is
slightly rounded, the anal opening nearer the extremity of the tail than
the bag. The intestine is narrow, and the embryonic fin extending from the
vent to the yolk-bag is quite narrow. In a somewhat more advanced
stage,--hatched a few hours earlier,--the upper edge of the yolk-bag is
covered with black pigment cells, and minute black pigment cells appear on
the surface of the alimentary canal. There are no traces of embryonic
fin-rays either in this stage or the one preceding; the structure of the
embryonic fin is as in bony Fishes--previous to the appearance of these
embryonic fin-rays--finely granular. Seen in profile, the yolk-bag is
ovoid; as seen from above, it is flattened, rectangular in front, with
rounded corners, tapering to a rounded point towards the posterior
extremity, with re-entering sides."
Footnote 508: This statement of Agassiz does not correspond
with the dates on the specimens sent to us--a fact no doubt due
to the hatching not taking place at the same time for all the
larvæ.
We have figured an embryo of 11 millims. in length, shortly after hatching
(Plate 34, fig. 12), the most important characters of which are as
follows:--The yolk-sack, which has now become much reduced, forms an
appendage attached to the ventral surface of the body, and has a very
elongated form as compared with its shape just before hatching. The mouth,
as also noticed by Agassiz, has a very open form. It is (Plate 34, fig. 13,
_m._) more or less rhomboidal, and is bounded behind by the mandibular arch
(_mn._) and laterally by the superior maxillary processes (_s.mx_). In
front of the mouth is placed the suctorial disc (_s.d._), the central
papillæ of which are arranged in groups. The opercular fold (_h.op._) is
very large, covering the arches behind. A well-marked groove is present
between the mandibular and opercular arches, but so far as we can make out
it is not a remnant of the hyomandibular cleft.
The pectoral fins (Plate 34, fig. 12, _pc.f._) are very prominent
longitudinal ridges, which, owing to their being placed on the surface of
the yolk-sack, project in a nearly vertical direction: a feature which is
also found in many Teleostean embryos with large yolk-sacks.
No traces of the pelvic fins have yet become developed.
The positions of the permanent dorsal, anal, and caudal fins, as pointed
out by Agassiz, are now indicated by a deposit of pigment in the embryonic
fin.
In an embryo on the sixth day after hatching, of about 15 millims. in
length, of which we have also given a figure (Plate 34, fig. 14), the
following fresh features deserve special notice.
In the region of the head there is a considerable elongation of the
pre-oral part, forming a short snout, at the end of which is placed the
suctorial disc. At the sides of the snout are placed the nasal pits, which
have become somewhat elongated anteriorly.
The mouth has lost its open rhomboidal shape, and has become greatly
narrowed in an antero-posterior direction, so that its opening is reduced
to a slit. The mandibles and maxillary processes are nearly parallel,
though both of them are very much shorter than in the adult. The operculum
is now a very large flap, and has extended so far backwards as to cover the
insertion of the pectoral fin. The two opercular folds nearly meet
ventrally.
The yolk-sack is still more reduced in size, one important consequence of
which is that the pectoral fins (_pc.f._) appear to spring out more or less
horizontally from the sides of the body, and at the same time their
primitive line of attachment to the body becomes transformed from a
longitudinal to a more or less transverse one.
The first traces of the pelvic fins are now visible as slight longitudinal
projections near the hinder end of the yolk-sack (_pl.f._).
The pigmentation marking the regions of the permanent fins has become more
pronounced, and it is to be specially noted that the ventral part of the
caudal fin (the permanent caudal) is considerably more prominent than the
dorsal fin opposite to it.
The next changes, as Agassiz points out, "are mainly in the lengthening of
the snout; the increase in length both of the lower and upper jaw; the
concentration of the sucker of the sucking disc; and the adoption of the
general colouring of somewhat older Fish. The lobe of the pectoral has
become specially prominent, and the outline of the fins is now indicated by
a fine milky granulation. Seen from above, the gill-cover is seen to leave
a large circular opening leading to the gill-arches, into which a current
of water is constantly passing, by the lateral expansion and contraction of
the gill-cover; the outer extremity of the gill-cover covers the base of
the pectorals. In a somewhat older stage the snout has become more
elongated, the sucker more concentrated, and the disproportionate size of
the terminal sucking-disc is reduced; the head, when seen from above,
becoming slightly elongated and pointed."
In a larva of about 18 days old and 21 millims. in length, of which we have
not given a figure, the snout has grown greatly in length, carrying with it
the nasal organs, the openings of which now appear to be divided into two
parts. The suctorial disc is still a prominent structure at the end of the
snout. The lower jaw has elongated correspondingly with the upper, so that
the gape is very considerable, though still very much less than in the
adult.
The opercular flaps overlap ventrally, the left being superficial. They
still cover the bases of the pectoral fins. The latter are described by
Agassiz as being "kept in constant rapid motion, so that the fleshy edge is
invisible, and the vibration seems almost involuntary, producing a constant
current round the opening leading into the cavity of the gills."
The pelvic fins are somewhat more prominent.
The yolk-sack, as pointed out by Agassiz, has now disappeared as an
external appendage.
After the stage last described the young Fish rapidly approaches the adult
form. To shew the changes effected we have figured the head of a larva of
about a month old and 23 millims. in length (Plate 34, fig. 15). The
suctorial disc, though much reduced, is still prominent at the end of the
snout. Eventually, as shewn by Agassiz, it forms the fleshy globular
termination of the upper jaw.
The most notable feature in which the larva now differs in its external
form from the adult is in the presence of an externally heterocercal tail,
caused by the persistence of the primitive caudal fin as an elongated
filament projecting beyond the permanent caudal (Plate 41, fig. 68).
Delicate dermal fin-rays are now conspicuous in the peripheral parts of all
the permanent fins. These rays closely resemble the horny fin-rays in the
fins of embryo Elasmobranchii in their development and structure. They
appear gradually to enlarge to form the permanent rays, and we have
followed out some of the stages of their growth, which is in many respects
interesting. Our observations are not, however, complete enough to publish,
and we can only say here that their early development and structure proves
their homology with the horny fibres or rays in fins of Elasmobranchii. The
skin is still, however, entirely naked, and without a trace of its future
armour of enamelled scales.
The tail of a much older larva, 11 centims. in length, in which the scales
have begun to be formed, is shewn in Plate 34, fig. 16.
We complete this section of our memoir by quoting the following passages
from Agassiz as to the habits of the young fish at the stages last
described:--
"In the stages intervening between plate iii, fig. 19, and plate iii,
fig. 30, the young _Lepidosteus_ frequently swim about, and become
readily separated from their point of attachment. In the stage of
plate iii, fig. 30, they remain often perfectly quiet close to the
surface of the water; but, when disturbed, move very rapidly about
through the water.... The young already have also the peculiar habit
of the adult of coming to the surface to swallow air. When they go
through the process under water of discharging air again they open
their jaws wide, and spread their gill-covers, and swallow as if they
were choking, making violent efforts, until a minute bubble of air has
become liberated, when they remain quiet again. The resemblance to a
Sturgeon in the general appearance of this stage of the young
_Lepidosteus_ is quite marked."
BRAIN.
I. _Anatomy._
The brain of _Lepidosteus_ has been figured by Busch (whose figure has been
copied by Miklucho-Maclay, and apparently by Huxley), by Owen and by Wilder
(No. 15). The figure of the latter author, representing a longitudinal
section through the brain, is the most satisfactory, the other figures
being in many respects inaccurate; but even Wilder's figure and
description, though taken from the fresh object, appear to us in some
respects inadequate. He offers, moreover, fresh interpretations of certain
parts of the brain which we shall discuss in the sequel.
We have examined two brains which, though extremely soft, were,
nevertheless, sufficiently well preserved to enable us to study the
external form. We have, moreover, made a complete series of transverse
sections through one of the brains, and our sections, though utterly
valueless from a histological point of view, have thrown some light on the
topographical anatomy of the brain.
Plate 38, figs. 47A, B, and C, represent three views of the brain, viz.:
from the side, from above, and from below. We will follow in our
description the usual division of the brain into fore-brain, mid-brain, and
hind-brain.
The fore-brain consists of an anterior portion forming the cerebrum, and a
posterior portion constituting the thalamencephalon.
The cerebrum at first sight appears to be composed of (_a_) a pair of
posterior and somewhat dorsal lobes, forming what have usually been
regarded as the true cerebral hemispheres, but called by Wilder the
prothalami, and (_b_) a pair of anterior and ventral lobes, usually
regarded as the olfactory lobes, from which the olfactory nerves spring.
Mainly from a comparison with our embryonic brains described in the sequel,
we are inclined to think that the usual interpretations are not wholly
correct, but that the true olfactory lobes are to be sought for in small
enlargements (Plate 38. figs. 47A, B, and C, _olf._) at the front end of
the brain[509] from which the olfactory nerves spring. The cerebrum proper
would then consist of a pair of anterior and ventral lobes (_ce._), and of
a pair of posterior lobes (_ce´._), both pairs uniting to form a basal
portion behind.
Footnote 509: The homologies of the olfactory lobes throughout
the group of Fishes require further investigation.
The two pairs of lobes probably correspond with the two parts of the
cerebrum of the Frog, the anterior of which, like that of _Lepidosteus_,
was held to be the olfactory lobe, till Götte's researches shewed that this
view was not tenable.
The anterior lobes of the cerebrum have a conical form, tapering
anteriorly, and are completely separated from each other. The posterior
lobes, as is best shewn in side views, have a semicircular form. Viewed
from above they appear as rounded prominences, and their dorsal surface is
marked by two conspicuous furrows (Plate 38, fig. 47B, _ce´._), which have
been noticed by Wilder, and are similar to those present in many Teleostei.
Their front ends overhang the base of the anterior cerebral lobes. The
basal portion of the cerebrum is an undivided lobe, the anterior wall of
which forms the lamina terminalis.
What we have above described as the posterior cerebral lobes have been
described by Wilder as constituting the everted dorsal border of the basal
portion of the cerebrum.
The portion of the cerebro-spinal canal within the cerebrum presents
certain primitive characters, which are in some respects dissimilar to
those of higher types, and have led Wilder to hold the posterior cerebral
lobes, together with what we have called the basal portion of the cerebrum,
to be structures peculiar to Fishes, for which he has proposed the name
"prothalami."
In the basal portion of the cerebrum there is an unpaired slit-shaped
ventricle, the outer walls of which are very thick. It is provided with a
floor formed of nervous matter, in part of which, judging from Wilder's
description, a well-marked commissure is placed. We have found in the larva
a large commissure in this situation (Plate 37, figs. 44 and 45, _a.c._);
and it may be regarded as the homologue of the anterior commissure of
higher types. This part of the ventricle is stated by Wilder to be without
a roof. This appears to us highly improbable. We could not, however,
determine the nature of the roof from our badly preserved specimens, but if
present, there is no doubt that it is extremely thin, as indeed it is in
the larva (Plate 37, fig. 46B). In a dorsal direction the unpaired
ventricle extends so as to separate the two posterior cerebral lobes.
Anteriorly the ventricle is prolonged into two horns, which penetrate for a
short distance, as _the lateral ventricles_, into the base of the anterior
cerebral lobes. The front part of each anterior cerebral lobe, as well as
of the whole of the posterior lobes, appears solid in our sections; but
Wilder describes the anterior horns of the ventricle as being prolonged for
the whole length of the anterior lobes.
In the embryos of all Vertebrates the cerebrum is not at first divided into
two lobes, so that the fact of the posterior part of the cerebrum in
_Lepidosteus_ and probably other Ganoids remaining permanently in the
undivided condition does not appear to us a sufficient ground for giving to
the lobes of this part of the cerebrum the special name of prothalami, as
proposed by Wilder, or for regarding them as a section of the brain
peculiar to Fishes.
The thalamencephalon (_th._) contains the usual parts, but is in some
respects peculiar. Its lateral walls, forming the optic thalami, are thick,
and are not sharply separated in front from the basal part of the cerebrum;
between them is placed the third ventricle. The thalami are of considerable
extent, though partially covered by the optic lobes and the posterior lobes
of the cerebrum. They are not, however, relatively so large as in other
Ganoid forms, more especially the Chondrostei and _Polypterus_.
On the roof of the thalamencephalon is placed a large thin-walled vesicle
(Plate 38, figs. 47A and B, _v.th._), which undoubtedly forms the most
characteristic structure connected with this part of the brain. Owing to
the wretched state of preservation of the specimens, we have found it
impossible to determine the exact relations of this body to the remainder
of the thalamencephalon; but it appears to be attached to the roof of the
thalamencephalon by a narrow stalk only. It extends forwards so as to
overlap part of the cerebrum in front, and is closely invested by a highly
vascular layer of the pia mater.
No mention is made by Wilder of this body; nor is it represented in his
figures or in those of the other anatomists who have given drawings of the
brain of _Lepidosteus_. It might at first be interpreted as a
highly-developed pineal gland, but a comparison with the brain of the larva
(vide p. 764) shews that this is not the case, but that the body in
question is represented in the larva by a special outgrowth of the roof of
the thalamencephalon. The vesicle of the roof of the thalamencephalon is
therefore to be regarded as a peculiar development of the tela choroidea of
the third ventricle.
How far this vesicle has a homologue in the brains of other Ganoids is not
certain, since negative evidence on this subject is all but valueless. It
is possible that a vesicular sack covering over the third ventricle of the
Sturgeon described by Stannius[510], and stated by him to be wholly formed
of the membranes of the brain, is really the homologue of our vesicle.
Footnote 510: "Ueb. d. Gehirn des Störs," Müller's _Archiv_,
1843, and _Lehrbuch d. vergl. Anat. d. Wirbelthiere_. Cattie,
_Archives de Biologie_, Vol. III. 1882, has recently described
in _Acipenser sturio_ a vesicle on the roof of the
thalamencephalon, whose cavity is continuous with the third
ventricle. This vesicle is clearly homologous with that in
_Lepidosteus_. (June 28, 1882.)
Wiedersheim[511] has recently described in _Protopterus_ a body which is
undoubtedly homologous with our vesicle, which he describes in the
following way:--
"Dorsalwärts ist das Zwischenhirn durch ein tiefes, von Hirnschlitz
eingenommenes Thal von Vorderhirn abgesetzt; dasselbe ist jedoch durch
eine häutige, mit der Pia mater zusammenhängende Kuppel oder Kapsel
überbrückt."
Footnote 511: R. Wiedersheim, _Morphol. Studien_, 1880, p.
71.
This "Kuppel" has precisely the same relations and a very similar
appearance to our vesicle. The true pineal gland is placed behind it. It
appears to us possible that the body found by Huxley[512] in _Ceratodus_,
which he holds to be the pineal gland, is in reality this vesicle. It is
moreover possible that what has usually been regarded as the pineal gland
in _Petromyzon_ may in reality be the homologue of the vesicle we have
found in _Lepidosteus_.
Footnote 512: "On _Ceratodus Forsteri_," &c., _Proc. Zool.
Soc._ 1876.
We have no observations on the pineal gland of the adult, but must refer
the reader for the structure and relations of this body to the
embryological section.
The infundibulum (Plate 38, fig. 47A, _in._) is very elongated. Immediately
in front of it is placed the optic chiasma (Plate 38, figs. 47A and C,
_op.ch._) from which the optic fibres can be traced passing along the sides
of the optic thalami and to the optic lobes, very much as in Müller's
figure of the brain of _Polypterus_.
On the sides of the infundibulum are placed two prominent bodies, the lobi
inferiores (_l.in._), each of which contains a cavity continuous with the
prolongation of the third ventricle into the infundibulum. The apex of the
infundibulum is enlarged, and to it is attached a pituitary body (_pt._).
The mid-brain is of considerable size, and consists of a basal portion
connecting the optic thalami with the medulla, and a pair of large optic
lobes (_op.l._). The iter a tertio ad quartum ventriculum, which forms the
ventricle of this part of the brain, is prolonged into each optic lobe, and
the floor of each prolongation is taken up by a dome-shaped projection, the
homologue of the torus semicircularis of Teleostei.
The hind-brain consists of the usual parts, the medulla oblongata and the
cerebellum. The medulla presents no peculiar features. The sides of the
fourth ventricle are thickened and everted, and marked with peculiar folds
(Plate 38, figs. 47A and B, _m.o._).
The cerebellum is much larger than in the majority of Ganoids, and
resembles in all essential features the cerebellum of Teleostei. In side
views it has a somewhat S-shaped form, from the presence of a peculiar
lateral sulcus (Plate 38, fig. 47A, _cb._). As shewn by Wilder, its wall
actually has in longitudinal section this form of curvature, owing to its
anterior part projecting forwards into the cavity of the iter[513]. This
forward projection is not, however, so conspicuous as in most Teleostei.
The cerebellum contains a large unpaired prolongation of the fourth
ventricle.
Footnote 513: In Wilder's figure the walls of the cerebellum
are represented as much too thin.
II. _Development._
The early development of the brain has already been described; and,
although we do not propose to give any detailed account of the later stages
of its growth, we have thought it worth while calling attention to certain
developmental features which may probably be regarded as to some extent
characteristic of the Ganoids. With this view we have figured (Plate 37,
figs. 44, 45) longitudinal sections of the brain at two stages, viz.: of
larvæ of 15 and 26 millims., and transverse sections (Plate 37, figs.
46A-G) of the brain of a larva at about the latter stage (25 millims.).
The original embryonic fore-brain is divided in both embryos into a
cerebrum (_ce._) in front and a thalamencephalon (_th_) behind. In the
younger embryo the cerebrum is a single lobe, as it is in the brains of all
Vertebrate embryos; but in the older larva it is anteriorly (Plate 37,
fig. 46A) completely divided into two hemispheres. The roof of the
undivided posterior part of the cerebrum is extremely thin (Plate 37, fig.
46B). Near the posterior border of the base of the cerebrum there is a
great development of nervous fibres, which may probably be regarded as in
part equivalent to the anterior commissure (Plate 37, figs. 44, 45 _a.c._).
Even in the oldest of the two brains the olfactory lobes are very slightly
developed, constituting, however, small lateral and ventral prominences of
the front end of the hemispheres. From each of them there springs a long
olfactory nerve, extending for the whole length of the rostrum to the
olfactory sack.
The thalamencephalon presents a very curious structure, and is relatively a
more important part of the brain than in the embryo of any other form which
we know of. Its roof, instead of being, as usual, compressed
antero-posteriorly[514], so as to be almost concealed between the cerebral
hemispheres and the optic lobes (mid-brain), projects on the surface for a
length quite equal to that of the cerebral hemispheres (Plate 37, figs. 44
and 45, _th._). In the median line the roof of the thalamencephalon is thin
and folded; at its posterior border is placed the opening of the small
pineal gland. This body is a papilliform process of the nervous matter of
the roof of this part of the brain, and instead of being directed forwards,
as in most Vertebrate types, tends somewhat backwards, and rests on the
mid-brain behind (Plate 37, figs. 44, 45, and 46C and D, _pn._). The roof
of the thalamencephalon immediately in front of the pineal gland forms a
sort of vesicle, the sides of which extend laterally as a pair of lobes,
shewn in transverse sections in Plate 37, figs. 46C and D, as _th.l._ This
vesicle becomes, we cannot doubt, the vesicle on the roof of the
thalamencephalon which we have described in the adult brain. Immediately in
front of the pineal gland the roof of the thalamencephalon contains a
transverse commissure (Plate 37, fig. 46C, _z._), which is the homologue of
a similarly situated commissure present in the Elasmobranch brain[515],
while behind the pineal gland is placed the posterior commissure. The sides
of the thalamencephalon are greatly thickened, forming the optic thalami
(Plate 37, figs. 46C and D, _op.th._), which are continuous in front with
the thickened outer walls of the hemispheres. Below, the thalamencephalon
is produced into a very elongated infundibulum (Plate 37, figs. 44, 45,
46E, _in._), the apex of which is trilobed as in Elasmobranchii and
Teleostei. The sides of the infundibulum exhibit two lobes, the lobi
inferiores (Plate 37, fig. 46E, _l.in._), which are continued posteriorly
into the crura cerebri.
Footnote 514: Vide F. M. Balfour, _Comparative Embryology_,
Vol. II. figs. 248 and 250.
Footnote 515: Vide F. M. Balfour, _Comparative Embryology_,
Vol. II. pp. 355-6 [the original edition], where it is
suggested that this commissure is the homologue of the grey
commissure of higher types.
The pituitary body[516] (Plate 37, figs. 44, 45, 46E, _pt._) is small, not
divided into lobes, and provided with a very minute lumen.
Footnote 516: We have not been able to work out the early
development of the pituitary body as satisfactorily as we could
have wished. In Plate 37, fig. 40, there is shewn an
invagination of the oral epithelium to form it; in Plate 37,
figs. 41 and 42, it is represented in transverse section in two
consecutive sections. Anteriorly it is still connected with the
oral epithelium (fig. 41), while posteriorly it is free. It is
possible that an earlier stage of it is shewn in Plate 36,
fig. 35. Were it not for the evidence in other types of its being
derived from the epiblast we should be inclined to regard it as
hypoblastic in origin.
In front of the infundibulum is the optic chiasma (Plate 37, fig. 46D,
_op.ch._), which is developed very early. It is, as stated by Müller, a
true chiasma.
The mid-brain (Plate 37, figs. 44 and 45, _m.b._) is large, and consists in
both stages of (1) a thickened floor forming the crura cerebri, the central
canal of which constitutes the iter a tertio ad quartum ventriculum; and
(2) the optic lobes (Plate 37, figs. 46E, F, G, _op.l._) above, each of
which is provided with a cavity continuous with the median iter. The optic
lobes are separated dorsally and in front by a well-marked median
longitudinal groove. Posteriorly they largely overlap the cerebellum. In
the anterior part of the optic lobes, at the point where the iter joins the
third ventricle, there may be seen slight projections of the floor into the
lumen of the optic lobes (Plate 37, fig. 46E). These masses probably become
in the adult the more conspicuous prominences of the floor of the
ventricles of the optic lobes, which we regard as homologous with the tori
semicirculares of the brain of the Teleostei.
The hind-brain is formed of the usual divisions, viz.: cerebellum and
medulla oblongata (Plate 37, figs. 44 and 45, _cb._, _md._). The former
constitutes a bilobed projection of the roof of the hind-brain. Only a
small portion of it is during these stages left uncovered by the optic
lobes, but the major part extends forwards for a considerable distance
under the optic lobes, as shewn in the transverse sections (Plate 37, figs.
46F and G, _cb._); and its two lobes, each with a prolongation of its
cavity, are continued forwards beyond the opening of the iter into the
fourth ventricle.
It is probable that the anterior horns of the cerebellum are equivalent to
the prolongations of the cerebellum into the central cavity of the optic
lobes of Teleostei, which are continuous with the so-called fornix of
Göttsche.
III. _Comparison of the larval and adult brain of Lepidosteus,
together with some observations on the systematic value
of the characters of the Ganoid brain._
The brain of the older of the two larvæ, which we have described,
sufficiently resembles in most of its features that of the adult to render
material assistance in the interpretation of certain of the parts of the
latter. It will be remembered that in the adult brain the parts usually
held to be olfactory lobes were described as the anterior cerebral lobes.
The grounds for this will be apparent by a comparison of the cerebrum of
the larva and adult. In the larva the cerebrum is formed of (1) an unpaired
basal portion with a thin roof, and (2) of a pair of anterior lobes, with
small olfactory bulbs at their free extremities.
The basal portion in the larva clearly corresponds in the adult with the
basal portion, together with the two posterior cerebral lobes, which are
merely special outgrowths of the dorsal edge of the primitive basal
portion. The pair of anterior lobes have exactly the same relations in the
larva as in the adult, except that in the former the ventricles are
prolonged for their whole length instead of being confined to their
proximal portions. If, therefore, our identifications of the larval parts
of the brain are correct, there can hardly be a question as to our
identifications of the parts in the adult. As concerns these
identifications, the comparison of the brain of our two larvæ appears
conclusive in favour of regarding the anterior lobes as parts of the
cerebrum, as distinguished from the olfactory lobes, in that they are
clearly derived from the undivided anterior portion of the cerebrum of the
younger larva.
The comparison of the larval brain with that of the adult again appears to
us to leave no doubt that the vesicle attached to the roof of the
thalamencephalon in the adult is the same structure as the bilobed
outgrowth of this roof in the larva; and since there is in addition a
well-developed pineal gland in the larva with the usual relations, there
can be no ground for identifying the vesicle in the adult with the pineal
gland.
Müller, in his often quoted memoir (No. 13), states that the brains of
Ganoids are peculiar and distinct from those both of Teleostei and
Elasmobranchii; but in addition to pointing out that the optic nerves form
a chiasma he does not particularly mention the features, to which he
alludes in general terms. More recently Wilder (No. 15) has returned to
this subject; and though, as we have already had occasion to point out, we
cannot accept all his identifications of the parts of the Ganoid brain, yet
he has called attention to certain characteristic features of the cerebrum
which have an undoubted systematic value.
The distinctive characters of the Ganoid brain are, in our opinion, (1) the
great elongation of the region of the thalamencephalon; and (2) the
unpaired condition of the posterior part of the cerebrum, and the presence
of so thin a roof to the ventricle of this part as to cause it to appear
open above.
The immense length of the region of the thalamencephalon is a feature in
the Ganoid brain which must at once strike any one who examines figures of
the brains of Chondrostei, _Polypterus_, or _Amia_. It is less striking in
the adult _Lepidosteus_, though here also we have shewn that the
thalamencephalon is really very greatly developed; but in the larva of
_Lepidosteus_ this feature is still better marked, so that the brain of the
larva may be described as being more characteristically Ganoid than that of
the adult.
The presence of a largely developed thalamencephalon at once distinguishes
a Ganoid brain from that of a Teleostean Fish, in which the optic thalami
are very much reduced; but _Lepidosteus_ shews its Teleostean affinities by
a commencing reduction of this part of the brain.
The large size of the thalamencephalon is also characteristic of the Ganoid
brain in comparison with the brain of the Dipnoi; but is not however so
very much more marked in the Ganoids than it is in some Elasmobranchii.
On the whole, we may consider the retention of a large thalamencephalon as
a primitive character.
The second feature which we have given as characteristic of the Ganoid
brain is essentially that which has been insisted upon by Wilder, though
somewhat differently expressed by him.
The simplest condition of the cerebrum is that found in the larva of
_Lepidosteus_, where there is an anterior pair of lobes, and an undivided
posterior portion with a simple prolongation of the third ventricle, and a
very thin roof. The dorsal edges of the posterior portion, adjoining the
thin roof, usually become somewhat everted (cf. Wilder), and in
_Lepidosteus_ these edges have in the adult a very great development, and
form (vide Plate 38, fig. 47A-C, _ce´._) two prominent lobes, which we have
spoken of as the posterior cerebral lobes.
These characters of the cerebrum are perhaps even more distinctive than
those of the thalamencephalon.
In Teleostei the cerebrum appears to be completely divided into two
hemispheres, which are, however, all but solid, the lateral ventricles
being only prolonged into their bases. In Dipnoi again there is either
(_Protopterus_, Wiedersheim[517]) a completely separated pair of oval
hemispheres, not unlike those of the lower Amphibia, or the oval
hemispheres are not completely separated from each other (_Ceratodus_,
Huxley[518], _Lepidosiren_, Hyrtl[519]); in either case the hemispheres are
traversed for the whole length by lateral ventricles which are either
completely or nearly completely separated from each other.
Footnote 517: _Morphol. Studien_, III. Jena, 1880.
Footnote 518: "On _Ceratodus Forsteri_," _Proc. Zool. Soc._
1876.
Footnote 519: _Lepidosiren paradoxa._ Prag. 1845.
In Elasmobranchii the cerebrum is an unpaired though bilobed body, but
traversed by two completely separated lateral ventricles, and without a
trace of the peculiar membranous roof found in Ganoids.
Not less interesting than the distinguishing characters of the Ganoid brain
are those cerebral characters which indicate affinities between
_Lepidosteus_ and other groups. The most striking of these are, as might
have been anticipated, in the direction of the Teleostei.
Although the foremost division of the brain is very dissimilar in the two
groups, yet the hind-brain in many Ganoids and the mid-brain also in
_Lepidosteus_ approaches closely to the Teleostean type. The most essential
feature of the cerebellum in Teleostei is its prolongation forwards into
the ventricles of the optic vesicles as the valvula cerebelli. We have
already seen that there is a homologous part of the cerebellum in
_Lepidosteus_; Stannius also describes this part in the Sturgeon, but no
such part is represented in Müller's figure of the brain of _Polypterus_,
or described by him in the text.
The cerebellum is in most Ganoids relatively smaller, and this is even the
case with _Amia_; but the cerebellum of _Lepidosteus_ is hardly less bulky
than that of most Teleostei.
The presence of tori semicirculares on the floor of the mid-brain of
_Lepidosteus_ again undoubtedly indicates its affinities with the
Teleostei, and such processes are stated by Stannius to be absent in the
Sturgeon, and have not, so far as we are aware, been described in other
Ganoids. Lastly we may point to the presence of well-developed lobi
inferiores in the brain of _Lepidosteus_ as an undoubted Teleostean
character.
On the whole, the brain of _Lepidosteus_, though preserving its true Ganoid
characters, approaches more closely to the brain of the Teleostei than that
of any other Ganoid, including even _Amia_.
It is not easy to point elsewhere to such marked resemblances of the Ganoid
brain, as to the brain of the Teleostei.
The division of the cerebrum into anterior and posterior lobes, which is
found in _Lepidosteus_, probably reappears again, as already indicated, in
the higher Amphibia. The presence of the peculiar vesicle attached to the
roof of the thalamencephalon has its parallel in the brain of
_Protopterus_, and as pointing in the same direction a general similarity
in the appearance of the brain of _Polypterus_ to that of the Dipnoi may be
mentioned.
There appears to us to be in no points a close resemblance between the
brain of Ganoids and that of Elasmobranchii.
SENSE ORGANS.
_Olfactory organ._
_Development._--The nasal sacks first arise during the late embryonic
period in the form of a pair of thickened patches of the nervous layer of
the epiblast on the dorsal surface of the front end of the head (Plate 37,
fig. 39, _ol._). The patches very soon become partially invaginated; and a
small cavity is developed between them and the epidermic layer of the
epiblast (Plate 37, figs. 42 and 43, _ol._). Subsequently, the roof of this
space, formed by the epidermic layer of the epiblast, is either broken
through or absorbed; and thus open pits, _lined entirely by the nervous
layer of the epidermis_, are formed.
We are not acquainted with any description of an exactly similar mode of
origin of the olfactory pits, though the process is almost identical with
that of the other sense organs.
We have not worked out in detail the mode of formation of the double
openings of the olfactory pits, but there can be but little doubt that it
is caused by the division of the single opening into two.
The olfactory nerve is formed very early (Plate 37, fig. 39, I), and, as
Marshall has found in Aves and Elasmobranchii, it arises at a stage prior
to the first differentiation of an olfactory bulb as a special lobe of the
brain.
_The Eye._
_Anatomy._--We have not made a careful histological examination of the eye
of _Lepidosteus_, which in our specimens was not sufficiently well
preserved for such a purpose; but we have found a vascular membrane
enveloping the vitreous humour on its retinal aspect, which, so far as we
know, is unlike anything which has so far been met with in the eye of any
other adult Vertebrate.
The membrane itself is placed immediately outside the hyaloid membrane,
_i.e._ on the side of the hyaloid membrane bounding the vitreous humour. It
is easily removed from the retina, to which it is only adherent at the
entrance of the optic nerve. In both the eyes we examined it also adhered,
at one point, to the capsule of the lens, but we could not make out whether
this adhesion was natural, or artificially produced by the coagulation of a
thin layer of albuminous matter. In one instance, at any rate, the adhesion
appeared firmer than could easily be produced artificially.
The arrangement of the vessels in the membrane is shewn diagrammatically in
Plate 38, fig. 49, while the characteristic form of the capillary plexus is
represented in Plate 38, fig. 50.
The arterial supply appears to be derived from a vessel perforating the
retina close to the optic nerve, and obviously homologous with the artery
of the processus falciformis and pecten of Teleostei and Birds, and with
the arteria centralis retinæ of Mammals. From this vessel branches diverge
and pursue a course towards the periphery. They give off numerous branches,
the blood from which enters a capillary plexus (Plate 38, figs. 49 and 50)
and is collected again by veins, which pass outwards and finally bend over
and fall into (Plate 38, fig. 49) a circular vein (_cr.v._) placed at the
outer edge of the retina along the insertion of the iris (_ir_). The
terminal branches of some of the main arteries appear also to fall directly
into this vein.
The membrane supporting the vessels just described is composed of a
transparent matrix, in which numerous cells are embedded (Plate 38,
fig. 50).
_Development._--In the account of the first stages of development of
_Lepidosteus_, the mode of formation of the optic cup, the lens, &c., have
been described (vide Plates 35 and 36, figs. 23, 26, 35). With reference to
the later stages in the development of the eye, the only subject with which
we propose to deal is the growth of the mesoblastic processes which enter
the cavity of the vitreous humour through the choroid slit.
_Lepidosteus_ is very remarkable for the great number of mesoblast cells
which thus enter the cavity of the vitreous humour, and for the fact that
these cells are _at first unaccompanied by any vascular structures_ (Plate
37, fig. 43, _v.h_). The mesoblast cells are scattered through the
vitreous humour, and there can be no doubt that during early larval life,
at a period however when the larva is certainly able to see, every
histologist would consider the vitreous humour to be a tissue formed of
scattered cells, with a large amount of intercellular substance; and the
fact that it is so appears to us to demonstrate that Kessler's view of the
vitreous humour being a mere transudation is not tenable.
In the larva five or six days after hatching, and about 15 millims. in
length, the choroid slit is open for its whole length. The edges of the
slit near the lens are folded, so as to form a ridge projecting into the
cavity of the vitreous humour, while nearer the insertion of the optic
nerve they cease to exhibit any such structure. The mesoblast, though it
projects between the lips of the ridge near the lens, only extends through
the choroid slit into the cavity of the vitreous humour in the
neighbourhood of the optic nerve. Here it forms a lamina with a thickened
edge, from which scattered cells in the cavity of the vitreous humour seem
to radiate.
At a slightly later stage than that just described, blood-vessels become
developed within the cavity of the vitreous humour, and form the vascular
membrane already described in the adult, placed close to the layer of
nerve-fibres of the retina, but separated from this layer by the hyaloid
membrane (Plate 38, fig. 48, _v.sh._). The artery bringing the blood to the
above vascular membrane is bound up in the same sheath as the optic nerve,
and passes through the choroid slit very close to the optic nerve. Its
entrance into the cavity of the vitreous humour is shewn in Plate 38,
fig. 48 (_vs._); its relation to the optic nerve in Plate 37, fig. 46, C
and D (_vs._).
The above sheath has, so far as we know, its nearest analogue in the eye of
_Alytes_, where, however, it is only found in the larva.
The reader who will take the trouble to refer to the account of the
imperfectly-developed processus falciformis of the Elasmobranch eye in the
treatise _On Comparative Embryology_, by one of us[520], will not fail to
recognize that the folds of the retina at the sides of the choroid slit,
and the mesoblastic process passing through this slit, are strikingly
similar in _Lepidosteus_ and Elasmobranchii; and that, if we are justified
in holding them to be an imperfectly-developed processus falciformis in the
one case, we are equally so in the other.
Footnote 520: Vol. II. p. 414 [the original edition].
Johannes Müller mentions the absence of a processus falciformis as one of
the features distinguishing Ganoids and Teleostei. So far as the systematic
separation of the two groups is concerned, he is probably perfectly
justified in this course; but it is interesting to notice that both in
Ganoids and Elasmobranchii we have traces of a structure which undergoes a
very special development in the Teleostei, and that the processus
falciformis of Teleostei is therefore to be regarded, not as an organ
peculiar to them, but as the peculiar modification within the group of a
primitive Vertebrate organ.
SUCTORIAL DISC.
One of the most remarkable organs of the larval _Lepidosteus_ is the
suctorial disc, placed at the front end of the head, to which we have made
numerous allusions in the first section of this memoir.
The external features of the disc have been fully dealt with by Agassiz,
and he also explained its function by observations on the habits of the
larva. We have already quoted (p. 755) a passage from Agassiz' memoir
shewing how the young Fishes use the disc to attach themselves firmly to
any convenient object. The discs appear in fact to be highly efficient
organs of attachment, in that the young Fish can remain suspended by them
to the sides of the jar, even after the water has been lowered below the
level at which they are attached.
The disc is formed two or three days before hatching, and from Agassiz'
statements, it appears to come into use immediately the young Fish is
liberated from the egg membranes.
We have examined the histological structure of the disc at various ages of
its growth, and may refer the reader to Plate 34, figs. 11 and 13, and
Plate 37, figs. 40 and 44. The result of our examination has been to shew
that the disc is provided with a series of papillæ often exhibiting a
bilateral arrangement. The papillæ are mainly constituted of highly
modified cells of the mucous layer of the epidermis. These cells have the
form of elongated columns, the nucleus being placed at the base, and the
main mass of the cells being filled with a protoplasmic reticulum. They may
probably be regarded as modified mucous cells. In the mesoblast adjoining
the suctorial disc there are numerous sinus-like vascular channels.
It does not appear probable that the disc has a true sucking action. It is
unprovided with muscular elements, and there appears to be no mechanism by
which it could act as a sucking organ. We must suppose, therefore, that its
adhesive power depends upon the capacity of the cells composing its papillæ
to pour out a sticky secretion.
MUSCULAR SYSTEM.
There is a peculiarity in the muscular system of _Lepidosteus_, which so
far as we know has not been previously noticed. It is that the lateral
muscles of each side are not divided, either in the region of the trunk or
of the tail, into a dorso-lateral and ventro-lateral division.
This peculiarity is equally characteristic of the older larvæ as of the
adult, and is shewn in Plate 41, figs. 67, 72, and 73, and Plate 42, figs.
74-76. In the Cyclostomata the lateral muscles are not divided into dorsal
and ventral sections; but except in this group such a division has been
hitherto considered as invariable amongst Fishes.
This character must, without doubt, be held to be the indication of a very
primitive arrangement of the muscular system. In the embryos of all Fishes
with the usual type of the lateral muscles, the undivided condition of the
muscles precedes the divided condition; and in primitive forms such as the
Cyclostomata and Amphioxus the embryonic condition is retained, as it is in
_Lepidosteus_.
SKELETON.
PART I.--_Vertebral column and ribs of the adult._
A typical vertebra from the trunk of _Lepidosteus_ has the following
characters (Plate 42, figs. 80 and 81).
The centrum is slightly narrower in the middle than at its two extremities.
It articulates with adjacent vertebræ by a convex face in front and a
concave face behind, being thus, according to Owen's nomenclature,
opisthocoelous. It presents on its under surface a well-marked longitudinal
ridge, which in many vertebræ is only united at its two extremities with
the main body of the vertebra.
From the lateral borders of the centrum there project, at a point slightly
nearer the front than the hind end, a pair of prominent hæmal processes
(_h.a._), to the ends of which are articulated the ribs. These processes
have a nearly horizontal direction in the greater part of the trunk, though
bent downwards in the tail.
The neural arches (_n.a._) have a somewhat complicated form. They are
mainly composed of two vertical plates, the breadth of the basal parts of
which is nearly as great as the length of the vertebræ, so that
comparatively narrow spaces are left between the neural arches of
successive vertebræ for the passage of the spinal nerves. Some little way
from its dorsal extremity each neural arch sends a horizontal process
inwards, which meets its fellow and so forms a roof for the spinal canal.
These processes appear to be confined to the posterior parts of the
vertebræ, so that at the front ends of the vertebræ, and in the spaces
between them, the neural canal is without an osseous roof. Above the level
of this osseous roof there is a narrow passage, bounded laterally by the
dorsal extremities of the neural plates. This passage is mainly filled up
by a series of cartilaginous elements (Plate 42, figs. 80 and 81, _i.c._)
(probably fibro-cartilage), which rest upon the roof of the neural canal.
Each element is situated _intervertebrally_, its anterior end being wedged
in between the two dorsal processes of the neural arch of the vertebra in
front, and its posterior end extending for some distance over the vertebra
behind. The successive elements are connected by fibrous tissue, and are
continuous dorsally with a fibrous band, known as the ligamentum
longitudinale superius (Plate 42, figs. 80 and 81, _l.l._), characteristic
of Fishes generally, and running continuously for the whole length of the
vertebral column. Each of the cartilaginous elements is, as will be
afterwards shewn, developed as two independent pieces of cartilage, and
might be compared with the dorsal element which usually forms the keystone
of the neural arch in Elasmobranchii, were not the latter vertebral instead
of intervertebral in position. More or less similar elements are described
by Götte in the neural arches of many Teleostei, which also, however,
appear to be vertebrally placed, and he has compared them and the
corresponding elements in the Sturgeon with the Elasmobranch cartilages
forming the keystone of the neural arch. Götte does not, however, appear to
have distinguished between the cartilaginous elements, and the osseous
elements forming the roof of the spinal canal, which are true membrane
bones; it is probable that the two are not so clearly separated in other
types as in _Lepidosteus_.
The posterior ends of the neural plates of the neural arches are continued
into the dorsal processes directed obliquely upwards and backwards, which
have been somewhat unfortunately described by Stannius as rib-like
projections of the neural arch. The dorsal processes of the two sides do
not meet, but between them is placed a median free spinous element, also
directed obliquely upwards and backwards, which forms a kind of roof for
the groove in which the cartilaginous elements and the ligamentum
longitudinale are placed.
The vertebræ are wholly formed of a very cellular osseous tissue, in which
a distinction between the bases of the neural and hæmal processes and the
remainder of the vertebra is not recognizable. The bodies of the vertebræ
are, moreover, directly continuous with the neural and hæmal arches.
The ribs in the region of the trunk are articulated to the ends of the long
hæmal processes. They envelop the body-cavity, their proximal parts being
placed immediately outside the peritoneal membrane, along the bases of the
intermuscular septa. Their distal ends do not, however, remain close to the
peritoneal membrane, _but pass outwards along the intermuscular septa till
their free ends come into very close proximity with the skin_. This
peculiarity, which holds good in the adult for all the free ribs, is shewn
in one of the anterior ribs of an advanced larva in Plate 41, fig. 72
(_rb._). We are not aware that this has been previously noticed, but it
appears to us to be a point not without interest in all questions which
concern the homology of rib-like structures occupying different positions
in relation to the muscles. Its bearings are fully dealt with in the
section of this paper devoted to the consideration of the homologies of the
ribs in Fishes.
As regards the behaviour of the ribs in the transitional region between the
trunk and the tail, we cannot do better than translate the description
given by Gegenbaur of this region (No. 6, p. 411):--"Up to the 34th
vertebra the ribs borne by the laterally and posteriorly directed processes
present nothing remarkable, though they have gradually become shorter. The
ribs of the 35th vertebra exhibit a slight curvature outwards of their free
ends, a peculiarity still more marked in the 36th. The last named pair of
ribs converge somewhat in their descent backwards so that both ribs
decidedly approach before bending outwards. The 37th vertebra is no longer
provided with freely terminating ribs, but on the contrary, the same pair
of processes which in front was provided with ribs, bears a short forked
process as the hæmal arch. _The two, up to this point separated ribs, have
here formed a hæmal arch by the fusion of their lower ends, which arch is
movable just like the ribs, and, like them, is attached to the vertebral
column._"
In the region of the tail-fin the hæmal arches supporting the caudal
fin-rays are very much enlarged.
PART II.--_Development of the vertebral column and ribs._
The first development and early histological changes of the notochord have
already been given, and we may take up the history of the vertebral column
at a period when the notochord forms a large circular rod, whose cells are
already highly vacuolated, while the septa between the vacuoles form a
delicate wide-meshed reticulum. Surrounding the notochord is the usual
cuticular sheath, which is still thin.
The first indications of the future vertebral column are to be found in the
formation of a distinct mesoblastic investment of the notochord. On the
dorsal aspect of the notochord, the mesoblast forms two ridges, one on each
side, which are prolonged upwards so as to meet above the neural canal, for
which they form a kind of sheath. On the ventral side of the notochord
there are also two ridges, which are, however, except on the tail, much
less prominent than the dorsal ridges.
The changes which next ensue are practically identical with those which
take place in Teleostei. Around the cuticular sheath of the notochord there
is formed an elastic membrane--the membrana elastica externa. At the same
time the basal parts of the dorsal, or as we may perhaps more conveniently
call them, the neural ridges of the notochord become enlarged at each
intermuscular septum, and the tissue of these enlargements soon becomes
converted into cartilage, thus forming a series of independent paired
neural processes riding on the membrana elastica externa surrounding the
notochord, and extending about two-thirds of the way up the sides of the
medullary cord. They are shewn in transverse section in Plate 41, fig. 67
(_n.a._), and in a side view in fig. 68 (_n.a._).
Simultaneously with the neural arches, the hæmal arches also become
established, and arise by the formation of similar enlargements of the
ventral or hæmal ridges. In the trunk they are very small, but in the
region of the tail their condition is very different. At the front end of
the anal fin the paired hæmal arches suddenly enlarge and extend
ventralwards (Plate 41, fig. 67, _h.a._).
Each succeeding pair of arches becomes larger than the one in front, and
the two elements of each arch first nearly meet below the caudal vein
(Plate 41, fig. 67) and finally actually do so, forming in this way a
completely closed hæmal canal. At the point where they first meet the
permanent caudal fin commences, and here (Plate 41, fig. 68) we find that
not only do the hæmal arches meet and coalesce below the caudal vein, but
they are actually produced into long spines supporting the fin-rays of the
caudal fin, which thus differs from the other fins in being supported by
parts of the true vertebral column and not by independently formed elements
of the skeleton.
Each of the large caudal hæmal arches, including the spine, forms a
continuous whole, and arises at an earlier period of larval life than any
other part of the vertebral column. We noticed the first indications of the
neural arches in the larva of about a week old, while they are converted
into fully formed cartilage in the larva of three weeks.
The neural and hæmal arches, resting on the membrana elastica externa, do
not at this early stage in the least constrict the notochord. They grow
gradually more definite, till the larva is five or six weeks old and about
26 millims. in length, but otherwise for a long time undergo no important
changes. During the same period, however, the true sheath of the notochord
greatly increases in thickness, and the membrana elastica externa becomes
more definite. So far it would be impossible to distinguish the development
of the vertebral column of _Lepidosteus_ from that of a Teleostean Fish.
Of the stages immediately following we have unfortunately had no examples,
but we have been fortunate enough to obtain some young specimens of
_Lepidosteus_[521], which have enabled us to work out with tolerable
completeness the remainder of the developmental history of the vertebral
column. In the next oldest larva, of about 5.5 centims., the changes which
have taken place are already sufficient to differentiate the vertebral
column of _Lepidosteus_ from that of a Teleostean, and to shew how certain
of the characteristic features of the adult take their origin.
Footnote 521: These specimens were given to us by Professor W.
K. Parker, who received them from Professor Burt G. Wilder.
In the notochord the most important and striking change consists in the
appearance of a series of very well marked vertebral constrictions
_opposite the insertions of the neural and hæmal arches_. The first
constrictions of the notochord are thus, as in other Fishes, vertebral; and
although, owing to the growth of the intervertebral cartilage, the
vertebral constrictions are subsequently replaced by intervertebral
constrictions, yet at the same time the primitive occurrence of vertebral
constrictions demonstrates that the vertebral column of _Lepidosteus_ is a
modification of a type of vertebral column with biconcave vertebræ.
The structure of the gelatinous body of the notochord has undergone no
important change. The sheath, however, exhibits certain features which
deserve careful description. In the first place the attention of the
observer is at once struck by the fact that, in the vertebral regions, the
sheath is much thicker (.014 millim.) than in the intervertebral (.005
millim.), and a careful examination of the sheath in longitudinal sections
shews that the thickening is due to the special differentiation of a
superficial part (Plate 41, fig. 69, _sh._) of the sheath in each vertebral
region. This part is somewhat granular as compared to the remainder,
especially in longitudinal sections. It forms a cylinder (the wall of which
is about .01 millim. thick) in each vertebral region, immediately within
the membrana elastica externa. Between it and the gelatinous tissue of the
notochord within there is a very thin unmodified portion of the sheath,
which is continuous with the thinner intervertebral parts of the sheath.
This part of the sheath is faintly, but at the same time distinctly,
concentrically striated--a probable indication of concentric fibres. The
inner unmodified layer of the sheath has the appearance in transverse
sections through the vertebral regions of an inner membrane, and may
perhaps be Kölliker's "membrana elastica interna."
We are not aware that any similar modification of the sheath has been
described in other forms.
The whole sheath is still invested by a very distinct membrana elastica
externa (_m.el_).
The changes which have taken place in the parts which form the permanent
vertebræ will be best understood from Plate 41, figs. 69-71. From the
transverse section (fig. 70) it will be seen that there are still neural
and hæmal arches resting upon the membrana elastica externa; but
longitudinal sections (fig. 69) shew that laterally these arches join a
cartilaginous tube, embracing the intervertebral regions of the notochord,
and continuous from one vertebra to the next.
It will be convenient to treat separately the neural arches, the hæmal
arches with their appendages, and the intervertebral cartilaginous rings.
The neural arches, except in the fact of embracing a relatively smaller
part of the neural tube than in the earlier stage, do not at first sight
appear to have undergone any changes. Viewed from the side, however, in
dissected specimens, they are seen to be prolonged upwards so as to unite
above with bars of cartilage directed obliquely backwards. An explanation
of this appearance is easily found in the sections. The cartilaginous
neural arches are invested by a delicate layer of homogeneous bone,
developed in the perichondrium, and this bone is prolonged beyond the
cartilage and joins a similar osseous investment of the dorsal bars above
mentioned. The whole of these parts may, it appears to us, be certainly
reckoned as parts of the neural arches, so that at this stage each neural
arch consists of: (1) a pair of basal portions resting on the notochord
consisting of cartilage invested by bone, (2) of a pair of dorsal
cartilaginous bars invested in bone (_n.a´._), and (3) of osseous bars
connecting (1) and (2).
Though, in the absence of the immediately preceding stages, it is not
perfectly certain that the dorsal pieces of cartilage are developed
independently of the ventral, there appears to us every probability that
this is so; and thus the cartilage of each neural arch is developed
discontinuously, while the permanent bony neural arch, which commences as a
deposit of bone partly in the perichondrium and partly in the intervening
membrane, forms a continuous structure.
Analogous occurrences have been described by Götte in Teleostei.
The dorsal portion of each neural arch becomes what we have called the
dorsal process of the adult arch.
Between the dorsal processes of the two sides there is placed a median rod
of cartilage (Plate 41, fig. 70, _i.s._), which in its development is
wholly independent of the true neural arches, and which constitutes the
median spinous element of the adult. In tracing these backwards it becomes
obvious that they are homologous with the interspinous elements supporting
the dorsal fin, in that they are replaced by these interspinous elements in
the region of the dorsal fin, and that the interspinous bones occupy the
same position as the median spinous processes. This homology was first
pointed out by Götte in the case of the Teleostei.
Immediately beneath this rod is placed the longitudinal ligament (Plate 41,
fig. 70, _l.l._), but there is as yet no trace of a junction between the
neural arches of the two sides in the space between the longitudinal
ligament and the spinal cord.
The basal parts of the neural arches of the two sides are united dorsally
by a thin cartilaginous layer resting on the sheath of the notochord, but
they are not united ventrally with the hæmal arches.
The hæmal processes in the trunk are much more prominent than in the
preceding stage, and their bases are united ventrally by a tolerably thick
layer of cartilage. In the trunk they are continuous with the so-called
ribs of the adult (Plate 41, fig. 70); but in order to study the nature of
these ribs it is necessary to trace the modifications undergone by the
hæmal arches in passing from the tail to the trunk.
It will be remembered that at an earlier stage the hæmal arches in the
region of the tail-fin were fully formed, and that through the anterior
part of the caudal region the hæmal processes were far advanced in
development, and just in front of the caudal fin had actually met below the
caudal vein.
The mode of development of the hæmal arches in the tail as _unjointed_
cartilaginous bars investing the caudal arteries and veins is so similar to
that of the caudal hæmal arches of Elasmobranchii, that it appears to us
impossible to doubt their identity in the two groups[522].