THE EMBRYOLOGICAL CRITERION
Pander's work of 1817 was the forerunner of an embryological period in
which men's hopes and interest centred round the study of development.
"With bewilderment we saw ourselves transported to the strange soil of a
new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.
von Baer's _Entwickelungsgeschichte_ was by far the greatest product of
this time, but it stands in a measure apart; we have in this chapter to
consider the lesser men who were Baer's contemporaries, friends,
followers or critics.
It was largely a German science, this new embryology, and its leaders
were all personally acquainted. Pander, von Baer and Rathke were on
friendly terms with one another; von Baer dedicated his master-work to
Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von
Baer. Interest in the new science was, however, not confined to Germany.
In Italy, Rusconi commenced in 1817 his pioneer researches on the
development of the Amphibia with a _Descrizione anatomica degli organi
della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in
which he traced the metamorphoses of the aortic arches. This was
followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and
in 1826 by his memoir _Du developpement de la grenouille_ (Milan). In
this last paper he described how the dark upper hemisphere of the frog's
egg grows down over the lower white hemisphere and leaves free only the
yolk plug; he observed the segmentation cavity and the archenteron, but
thought that the former became the alimentary canal; he observed and
interpreted rightly the formation of the medullary folds. The circular
blastopore in the frog in later years often went by the name of the anus
of Rusconi.
In France Dutrochet[183] investigated the foetal membranes in various
vertebrate classes; Prevost and Dumas studied the very earliest stages
of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii.,
iii., 1824, xii., 1827).
A little later came Duges' studies of the osteology and myology of
developing amphibia (1834),[184] and Coste's careful researches into the
early developmental history of mammals.[185]
[Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)]
It was in 1825 that Heinrich Rathke (1793-1860), published his famous
discovery of gill-slits in the embryo of a mammal,[186] a discovery which
aroused considerable interest, and greatly stimulated embryological
research. He describes how in a young embryo of a pig he saw four slits
in the region of the neck, going right through into the oesophagus. They
were separated by partitions which he called _Kiemenbogen_
(gill-arches), and immediately in front of the first gill-slit lay the
developing lower jaw. He compared these gill-slits with those of a
dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV.,
fig. 1).
Later in the same year Rathke discovered gill-slits in the chick,[187] in
this case finding only three. He described growing out from in front of
the first slit a structure which he compared to the operculum or
gill-cover of a fish.
These discoveries were confirmed and extended for the chick[188] by the
embryologist Huschke, a pupil of Oken. Like Rathke, he found only three
indubitable gill-slits, but he noticed that the body-wall in front of
the first gill-slit was really composed of two arches, which were on the
whole similar to the gill-arches. The hinder of these two seemed to him
to be a horn of the hyoid, the front one, which was bent at an angle, to
be the rudiment of the upper and lower jaws (p. 401). Between these two
arches he found an opening, just as between two gill-arches a gill-slit.
This opening led into the mouth-cavity, and according to Huschke it
became the external ear-passage. He discovered also three pairs of
aortic arches in close relation with the gill-arches, so close indeed,
that he did not hesitate to call them gill-arteries, and to recognise
their resemblance with the aortic arches of fish. He traced, in part at
least, the metamorphosis which these aortic arches undergo. This part of
his discovery he developed in fuller detail in a paper of 1828,[189] in
which he gave some excellent figures.
Shortly after Huschke's first paper, von Baer published his views and
observations on this subject in a short memoir in Meckel's _Archiv_.[190]
In this paper he confirmed Rathke's discovery, and described the slits
and arches in the dog and the chick. Both Rathke and he found gill-slits
in the human embryo about this time (p. 557). There were generally
present, he found, four gill-slits, and, as Rathke had suggested, the
first gill-arch became the lower jaw. Von Baer also confirmed Rathke's
discovery of the operculum, assigning it, however, to the second
gill-arch. He refused to accept Huschke's derivation of the auditory
meatus from the first gill-slit. Von Baer saw what had escaped Rathke
and Huschke, that there were, not three nor four, but as many as five
aortic arches.
In his view of the metamorphosis of the aortic arches in the chick the
first two pairs disappeared completely, the third pair gave rise to the
arteries of the head and the fore-limbs, the right side of the fourth
arch became the aorta, the left half of the fourth and the right half of
the fifth arch became the pulmonary arteries, while the left half of the
fifth arch disappeared. This schema, which for the last three arches was
the same as Huschke's, von Baer upheld for the chick even in the second
volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it,
however, for mammals in the same volume (p. 212), deriving both
pulmonary arteries from the fifth arch, and the aorta from the fourth
left. He fully recognised the great analogy of the embryonic arrangement
of gill-arches and gill-arteries in Tetrapoda with their arrangement in
fish (i., pp. 53, 73).
Huschke, in a paper of 1832,[191] chiefly devoted to the development of
the eye, figured and described the developing upper and lower jaws, and
maintained against von Baer that the first slit turns into the auditory
meatus and the Eustachian tube.
These were the first papers of the embryological period. Before going on
to discuss the principles which guided embryological research during the
next ten or twenty years it is convenient to note what were the main
lines of work characterising the period.
The typical figure of the period is Rathke, who produced a great deal of
first-class embryological work. He was, even more than von Baer, a
comparative embryologist, and there were few groups of animals that he
did not study. His first large publication, the _Beitraege zur Geschichte
der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work
in addition to the purely embryological; he commenced here his series of
papers on the development of the genital and urinary organs, continued
in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in
this line was Johannes Mueller, whose _Bildungsgeschichte der Genitalien_
(Duesseldorf) appeared in 1830.
In a memoir on the development of the crayfish which appeared in
1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer
theory propounded by Pander and von Baer. He was greatly struck by the
inverted position of the embryo with respect to the yolk. In following
out the development of the appendages he noticed how much alike were
jaws and legs in their earliest stage, and how this supported Savigny's
contention that the limbs of Arthropods belonged to one single type of
structure. In his paper (1832) on the development of the fresh-water
Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the
original similarity in development of antennae, jaws and legs, he writes,
"Whatever the doubts one may have reserved as to the intimate relation
existing between the jaws and feet of articulate animals after the
researches of Savigny on this subject and mine on developing crayfish,
they must all fall to the ground when one examines with care the
development of the fresh-water Asellus" (p. 147 of French translation).
Further comparative work by Rathke is found in the two volumes of
_Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus
Taurien_ (1837), which contains embryological studies of many different
types, including a study of the uniform plan of arthropod limbs. Later
on Rathke devoted himself more to vertebrate embryology, producing among
other works his classical papers on the development of the adder (1839),
of the tortoise (1848), and of the crocodile (1866). He laid the
foundations of all subsequent knowledge of the development of the
blood-vascular system in a series of papers of various dates from 1838
to 1856. The diagrams in his paper on the aortic arches of reptiles
(1856) were for long copied in every text-book.
Rathke was a foremost worker in another important line of embryological
work, the study of the development of the skeleton and particularly of
the skull. We shall discuss the history of the embryological study of
the skull in some detail below; meantime, we note the two other
important lines of research which characterise this period. One is the
intensive study of the development of the human embryo, a study pursued
by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber,
Mueller, and Wharton Jones.[194] The other important line--the early
development of the Mammalia--was worked chiefly by Valentin,[195]
Coste,[196] and, above all, by Bischoff, whose series of papers[197] was
justly recognised as classical.
What interests us chiefly in the work of this embryological period is,
of course, the relation of embryology to comparative anatomy and to pure
morphology. The embryologists were not slow to see that their work threw
much light upon questions of homology, and upon the problem of the unity
of plan. Von Baer, we have seen, recognised this clearly in 1828;
Rathke, in one of his most brilliant papers, the
_Anatomisch-philosophische Untersuchungen ueber den Kiemenapparat und das
Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with
great effect to show the homology of the gill-arches and hyoid
throughout the vertebrate series; Johannes Mueller made great use of
embryology in his classical _Vergleichende Anatomie der Myxinoiden_ (i.
Theil, 1836), and, according to his pupil Reichert, firmly held the
opinion that embryology was the final court of appeal in disputed points
of comparative anatomy;[198] Reichert himself in a book of 1838
(_Vergleichende Entwickelungsgeschichte des Kopfes der nackten
Amphibien_) discussed the two different methods of arriving at the
"Type"--the anatomical method of comparing adults, and the embryological
method of comparing embryogenies. Of the embryological method, he says,
"Its aim is to distinguish during the formation of the organism the
originally given, the essence of the type, and to classify and interpret
what is added or altered in the further course of development.
Embryologists watch the gradual building up of the organism from its
foundations, and distinguish the fundament, the primordial form, the
type, from the individual developments; they reach thus, following
Nature in a certain measure, the essential structure of the organism,
and demonstrate the laws that manifest themselves during embryogeny" (p.
vi.). The embryologists, influenced in this greatly by von Baer,
gradually felt their way to substituting for the "Archetype" of pure
morphology what one may perhaps best call the _embryological archetype_.
How the transition was made we can best see by following out the course
of discovery in one particular line. We choose for this purpose the
development of the skull, a subject which excited much interest at this
time and upon which much quite fundamental work was done, particularly
by Rathke and Reichert.
Following up his discovery of gill-slits and arches in the embryos of
birds and mammals, Rathke in two papers of 1832[199] and 1833[200] worked
out the detailed homologies of the gill-arches in the higher
Vertebrates. He describes how in the embryo of the Blenny there is a
short, thick arch between the first gill-slit and the mouth. A furrow
appears down the middle of the arch dividing it incompletely into two.
In the anterior halves a cartilaginous rod is developed which is
connected with the skull; these rods become on either side the lower jaw
and "quadrate." In the posterior halves two similar rods are formed
which develop into the hyoid. The hyoid is at first connected with the
skull, but afterwards frees itself and becomes slung to the "quadrate."
From the hinder edge of the hyoid arch grows out the membranous
operculum, in which develop later the opercular bones and branchiostegal
rays. The upper jaw is an independent outgrowth of the serous layer.
The serial homology of the lower jaw and quadrate with the hyoid and
with the true gill-arches was thus established in fish, and Rathke had
little difficulty in demonstrating a similar origin of lower jaw and
hyoid in the embryos of higher Vertebrates. He could even, as we have
noted before, find the homologue of the operculum in a flap which grows
out from the hyoid arch in the embryo of birds.
But Rathke could not altogether shake himself free from the
transcendental notion of the homology of jaws with ribs, and this led
him to draw a certain distinction between the first two and the
remaining gill-arches, by which the homology of the former with the ribs
was asserted and the homology of the latter denied. He thought he could
show that the skeletal structures (lower jaw, "quadrate," and hyoid) of
the first two arches were formed in the serous layer, just like true
ribs, and like them in close connection with the vertebral skeletal
axis. The other, "true," gill-arches appeared to him to be formed in the
mucous layer, in the lining of the alimentary canal. They had no direct
connection with the vertebral column, and seemed therefore to belong to
what Carus[201] had called the visceral or splanchno-skeleton. He did not,
however, let this distinction hinder him from asserting the substantial
homology of all the gill-arches _inter se_, the first two included.
Rathke's discoveries relative to the development of the jaws, the hyoid
and the operculum, enabled him to make short work of the homologies
proposed for them by the transcendentalists. He could prove from
embryology that the jaws were not the equivalent of limbs, as so many
Okenians believed. He could reject, with a mere reference to the facts
of development, Geoffroy's comparison of the hyoid and the
branchiostegal rays in fish with sternum and ribs. He could show the
emptiness of the attempts made by Carus, Treviranus, de Blainville and
Geoffroy, to establish by anatomical comparison the homologies of the
opercular bones, for he could show that these bones were peculiar to
fish, and were scarcely indicated, and that only temporarily, in the
development of other Vertebrates.[202] He did not, however, himself
realise the relation of the ear-ossicles to the gill-arches, though he
knew that Spix and Geoffroy were quite wrong in homologising them with
the opercular bones in fish. He described, it is true, the development
of the external meatus of the ear and the Eustachian tube from the slit
which appears between the first and the second arch, as Huschke had done
before him; he described, in confirmation of Meckel, the "Meckelian
process" of the hammer running down inside the lower jaw; but the
discovery of the true homologies of the ear-ossicles was not made until
a year or two later by Reichert.
In his further study of the development of _Blennius viviparus_, Rathke
observed some important facts about the development of the vertebral
column and skull. He found that the vertebral centra were first formed
as rings in the chorda-sheath, which give off neural and haemal
processes. The vertebra later ossifies from four centres. The chorda
(notochord) is prolonged some little way into the head, and the base of
the cranium is formed by the expanded sheath, which reaches forward in
front of the end of the notochord. This cranial basis shows a division
into three segments, in which Rathke was inclined to see an indication
of three cranial vertebrae. (It turned out that this division into three
segments did not really exist, and Rathke later acknowledged that he had
made an error of observation.) The side walls of the skull grow out from
this base and form a fibrous capsule for the brain. The cranial section
of the chorda itself shows no sign of segmentation; but later on the
cranial portion of the chorda-sheath ossifies, like the vertebrae, from
several centres. The vomer, which, in the classical form of the
vertebral theory of the skull, was the centrum of the fourth, or
foremost, cranial vertebra, does not, according to Rathke, develop in
continuity with the cranial basis and the chorda sheath, but develops
separately in the facial region.
Von Baer, like Rathke at this time, was also to some extent a believer
in the vertebral theory of the skull. In his second volume (1834, pub.
1837) he holds that the development of the skull, as the sum of the
anterior vertebral arches, is in general the same as that of the other
neural arches, and is modified only by the great bulk of the brain
(_Entwickelungsgeschichte_, ii., p. 99). He had, however, some doubts as
to the entire correctness of the vertebral theory, doubts suggested by a
study of the developing skull. "In the course of the formation of the
head in the higher animals, something additional is introduced which
does not originally belong to the cranial vertebrae. At first we see the
vertebration in the hinder region of the skull very clearly. Afterwards
it becomes suddenly indistinct, as if some new formation overlaid it"
(i., p. 194).
Even more clearly is his doubt expressed in his paper on _Cyprinus_.
"Upon the formation of the vertebral column only this need be said, that
at this stage the notochord is very clearly seen, and the upper and
lower arches and spinous processes are visible right to the end of the
tail, but the separation into vertebrae ceases abruptly where the back
passes into the head. I do not hesitate to assert _that bony fish, too,
have at this stage an unsegmented cartilaginous cranium_ (as
cartilaginous fish have all their life), the prominences and hollows of
which constitute its only resemblance with the vertebral type" (1835, p.
19).
A convinced supporter of the vertebral theory was Johannes Mueller, who,
in his classical memoir on the Myxinoids,[203] discussed at some length
the relation between the development of the vertebrae and the development
of the skull. His memoir is principally devoted to comparative anatomy,
but in treating of the skeleton he pays much attention to development.
He describes the formation of the vertebrae in elasmobranch embryos; for
the facts regarding other Vertebrates he relies largely on work by
Rathke (_Blennius_, 1833) and Duges (1834). He recognises as the basis
of his comparisons the homology of the notochord in all vertebrate
embryos with the persistent notochord which forms the chief part or the
whole of the vertebral column in the Cyclostomes. The notochord
possesses an inner and an outer sheath and the outer sheath is
continuous with the _basis cranii_ (p. 92). It is in the outer sheath
that the vertebrae develop--from four separate pieces, in fish at least,
plus an additional element which helps to form the centrum. The skull of
Vertebrates consists, according to Mueller, of three vertebrae, whose
centra are the basioccipital, the basisphenoid and the presphenoid.
Other bones besides those belonging to the vertebrae are present, but
this formation out of three vertebrae gives the essential schema for the
skull. Now the brain capsule, like the sheath of the spinal cord, is a
development from the outer sheath of the notochord. If the skull
consists of vertebrae we should expect the centra of the skull-vertebrae
to develop in the outer sheath at the sides of the cranial section of
the notochord as two separate halves, just as do the bodies of the
vertebrae; we should expect further the cartilaginous side-walls of the
cranium to develop in the membranous brain-sheath just as the neural
arches develop in the membranous sheath of the spinal column. In
Rathke's discovery (!) of a segmentation of the _basis cranii_ into
three parts, and of the isolated formation of the vomer, Mueller sees a
confirmation of his view that the skull is composed of three and not
four vertebrae. But there is nothing in Rathke's observations to support
the idea that the centra of the cranial vertebrae are formed from
separate halves. Mueller has to be content with a reference to the state
of things in _Ammocoetes_ (which, by the way, he did not know to be the
young of _Petromyzon_). In the simple skull of _Ammocoetes_ the base is
formed chiefly by two cartilaginous bars lying more or less parallel
with the longitudinal axis of the skull and embracing with their hinder
ends the cranial portion of the notochord.
These bars, declares Mueller, are clearly the still separate halves of
the _pars basilaris cranii_, and represent the divided centra of the two
hinder cranial vertebrae. To complete the parallel between the
development of the skull and of the vertebrae, it would have been
necessary to show that the side walls of the cranium developed in a
similar manner from separate pieces. Mueller could not prove this point
from the available embryological data, and indeed the facts which he did
use had to be twisted to suit his theory. A curious apparent
confirmation of his idea that the centra of the cranial vertebrae are
formed from separate halves was supplied in 1839 by Rathke's discovery
of the trabeculae in the embryonic skull of the adder.
The next big step in the study of the development of the skull was
taken by a pupil of Mueller, C. B. Reichert, who showed in his work
very distinct traces of his master's influence. Reichert's first and
most important contribution to the subject was his paper on the
metamorphosis of the gill, or, as he called them, the visceral arches
in Vertebrates,[204] particularly in the two higher classes. Reichert
describes the similar origin in embryo of bird and mammal (pig) of
three "visceral" arches. These arches stand in close relation to the
three cranial vertebrae which Reichert, like Mueller, distinguishes. He
makes the retrograde step of admitting only three aortic arches, and
he is not inclined to consider the three visceral arches as equivalent
to the gill-arches of fish--in his opinion they have more analogy with
ribs, though differing somewhat from ribs in their later
modifications. The visceral arches are processes of the visceral
plates (von Baer), which grow downwards and meet in the middle line,
leaving between one another and the undivided body wall three visceral
slits opening into the pharynx. The first visceral process is
different in shape from the others, for it sends forward, parallel
with the head and at right angles to its downward portion, an upper
portion in which later the upper jaw is formed. The other two
processes are straight. From the hinder edge of the second visceral
arch there develops, as Rathke had seen, a fold which is comparable
with the operculum of fish. The first slit develops externally into
the ear-passage, internally into the Eustachian tube, and in the
middle a partition forms the tympanic ring and tympanum. Inside each
of the visceral processes on either side a cartilaginous rod develops.
In the first process this rod shows three segments, of which the first
lies inside that portion of the process which is parallel with the
head. This upper segment forms the foundation for the bones of the
upper jaw. The lowest segment of the cartilaginous rod becomes
Meckel's cartilage, and on the outer side of this the bones of the
lower jaw are formed. The middle segment becomes in mammals the incus
(one of the ear-ossicles), and in birds the quadrate. Meckel's
cartilage, which was discovered by Meckel[205] in fish, amphibians and
birds, is a long strip of cartilage which runs from the ear-ossicle
known as the hammer in mammals,[206] to the inside of the mandible.
Reichert shows how this relation comes about. The hammer, according to
his observations on the embryo of the pig, is simply the proximal end
of Meckel's cartilage, which later becomes separated off from the long
distal portion (see Fig. 9). The third ear-ossicle of mammals, the
stapes, comes not from the first arch but from the second. The
cartilaginous rod of the second arch segments like the first into
three pieces. Of these the uppermost disappears, the middle one, which
lies close up to the labyrinth of the ear, becomes the stapes, and the
lowest becomes the anterior horn of the hyoid. The stapes forms a
close connection with the hammer and the incus. In birds, where there
is a single ear-ossicle, the columella, the middle piece of arch I
forms, as we have seen, the quadrate, by means of which the lower jaw
is joined to the skull. The proximal end of Meckel's cartilage, which
in mammals forms the hammer, here gives the articular surface between
the lower jaw and the quadrate. The columella is formed from the
middle piece of the three into which the cartilage of the second arch
segments. It is, therefore, the homologue of the stapes in mammals.
The third arch takes a varying share, together with the second, in the
formation of the hyoid apparatus.
[Illustration: FIG. 9.--Meckel's Cartilage and Ear-ossicles in Embryo
of Pig. (After Reichert.)]
In this paper Reichert made a distinct advance on the previous workers
in the same field--Rathke, Huschke, von Baer, Martin St Ange, Duges.
Huschke was indeed the first to suggest that both upper and lower jaws
were formed in the first gill-arch. But both von Baer and Rathke[207] held
that the upper jaw developed as a special process independent of the
lower jaw rudiment, and the actual proof that the upper jaw is a
derivative of the first visceral arch seems to have been first supplied
by Reichert. His brilliant work on the development of the ear-ossicles
founded what we may justly call the classical theory of their
homologies. His views were attacked and in some points rectified, but
the main homologies he established are even now accepted by many,
perhaps the majority of morphologists.
In a paper of 1838 on the comparative embryology of the skull in
Amphibia,[208] Reichert added to his results for mammals and birds an
account of the fate of the first and second visceral arches in Anura and
Urodela.
The first visceral arch, he found, gave in Amphibia practically the same
structures as in the higher Vertebrates. Its skeleton segmented, as in
mammals and birds, into three parts; the upper part gave rise to the
palatine and pterygoid in Anura, but seemed to disappear in Urodeles,
where the so-called palatine and pterygoid developed in the mucous
membrane of the mouth; the middle part gave, as in birds, the quadrate,
which formed a suspensorium for both arches; the lower part, as Meckel's
cartilage, formed a foundation for the bones of the lower jaw. Of arch
II., the lower part became the horn of the hyoid, the upper part had a
varying fate. In some Anura it formed the ossicle of the ear (homologue
of the columella of birds and the stapes of mammals), in others it
disappeared. In reptiles the upper segment of the second arch formed, as
in birds, the columella.
The account of the metamorphoses of the visceral arches in Amphibia
forms only a small part of Reichert's memoir of 1838, the chief object
of which was to discover the general "typus" of the vertebrate skull,
and to follow out its modifications in the different classes. Von Baer
had shown that the generalised type appeared most clearly in the early
embryo; Reichert therefore sought the archetype of the skull in the
developing embryo. He brought to his task the preconceived notion that
the skull could be reduced to an assemblage of vertebrae, but he saw that
comparative anatomy alone could not effect this reduction; he had
recourse, therefore, to embryology, hoping to find in the simplified
structure of the embryo clear indications of three primitive cranial
vertebrae (p. 121, 1837).
In the head he distinguished two tubes, the upper formed by the dorsal
plates, the lower by the ventral or visceral plates. Both of these tubes
were derived from the serous or animal layer (_cf._ von Baer, _supra_,
p. 118). The walls of the lower tube were formed by the visceral
processes, within which later the skeleton of the visceral arches
developed. The walls of the upper tube formed the bones and muscles of
the cranium proper. The facial part of the head was formed by elements
from both upper and lower tubes. The dorsal tube showed signs of a
division into three cranial vertebrae (_Urwirbeln_, primitive vertebrae).
In mammals and birds, as Reichert had shown in his 1837 paper, the three
cranial vertebrae were indicated by transverse furrows on the ventral
surface of the still membranous skull (see Fig. 10, p. 148).
Even in mammals and birds, however, the positions of the eye, the
ear-labyrinth, and the three visceral arches were the safest guides to
the delimitation of the cranial vertebrae (pp. 134-138, 1837). In
Amphibia generally there were no definite lines of separation on the
skull itself. "At this stage," he writes of the cartilaginous cranium of
the frog, "we find no trace of a veritable division into vertebrae in the
cartilaginous trough formed by the _basis cranii_ and the side parts. On
the contrary, it is quite continuous, as it is also in the higher
Vertebrates during the process of chondrification" (p. 44, 1838). The
vertebrae in the membranous or cartilaginous skull could be delimited in
Amphibia by the help of the eye and the ear-labyrinth, which lie more or
less between the first and second, and the second and third vertebrae,
but, above all, by the vesicles of the brain.
As in the higher Vertebrates, the visceral arches are associated with
the cranial vertebrae as their ventral extensions, being equivalent to
the visceral plates which form the ventral portion of the "primitive
vertebrae" or primitive segments of the trunk.
[Illustration: FIG. 10.--Cranial Vertebrae and Visceral Arches in Embryo
of Pig. Ventral Aspect. (After Reichert.)]
If the three cranial vertebrae are not very distinct in the early stages
of development when the skull is still membranous or cartilaginous, they
become clearly delimited when ossification sets in. Three rings of bone
forming three more or less complete vertebrae are the final result of
ossification. The composition of these rings is as follows:--
+-------------------------------------------------------------------+
| | Base. | Sides. | Top. |
|----------------+---------------+-----------------+----------------|
|First vertebra |Presphenoid |Orbitosphenoids |Frontals |
| | | | |
|Second vertebra |Basisphenoid |Alisphenoids |Parietals |
| | | | |
|Third vertebra |Basioccipital |Exoccipitals |Supraoccipital |
+-------------------------------------------------------------------+
The other bones of the skull are not included in the vertebrae, and this
is in large part due to the fact that the sense capsules are formed
separately from the cranium (p. 29, 1838). The ear-labyrinth, it is
true, fuses indissolubly with the cranium at a later period, but the
bones which develop in its capsule are not for all that integral parts
of the primitive cranial vertebrae. This point, it is interesting to
note, had already been made by Oken in his _Programm_ (1807). But many
of the bones developed in relation to the sense organs can find their
place in the generalised embryonic schema or archetype of the vertebrate
skull, for they are of very constant occurrence during early
development.
Having arrived at a generalised embryonic type for the vertebrate skull,
of which the fundamental elements are the three cranial vertebrae and
their arches, Reichert goes on to discuss the particular forms under
which the skull appears in adult Vertebrates. He accepts in general von
Baer's law that the characters of the large groups appear earlier in
embryogeny than the characters of the lesser classificatory divisions.
"When we observe new and not originally present rudiments in very early
embryonic stages, as, for instance, that for the lacrymals, the
probability is that they belong to the distinctive development of one of
the _larger_ vertebrate groups. From these are to be carefully
distinguished such rudiments as arise later during ossification, mostly
as _ossa intercalaria_, in order to give greater strength to the skull
in view of the greater development of the brain, etc.; the latter give
their individual character to the _smaller_ vertebrate groups, and
comprise such bones as the _vomer_, the _Wormian bones_, the lowermost
turbinal, etc." (p. 63, 1838).
He did not accept the Meckel-Serres law of parallelism. He recognised
the great similarity between the unsegmented cartilaginous cranium of
Elasmobranchs, and the primordial cranium of the embryos of the higher
Vertebrates, but he did not think that the cranium of Elasmobranchs was
simply an undeveloped or embryonic stage of the skulls of the higher
forms. Rather "do the _Holocephala_, _Plagiostomata_, and _Cyclostomata_
appear to us to be lower developmental stages individually
differentiated, so that the other fully differentiated Vertebrates
cannot easily be referred directly to their type" (p. 152, 1838). The
skull of these lower fishes is itself a specialised one; it is an
individualised modification of a simple type of skull. And this holds
good in general of the skulls of the lower Vertebrates--they are
individualised exemplars of a simple general type, not merely unmodified
embryonic stages of the greatly differentiated skulls of the higher
Vertebrates (p. 250, 1838). Differentiation within the vertebrate phylum
is therefore not uniserial, but takes place in several directions.
Reichert describes two sorts of modifications of the typical
skull--class modifications and functional modifications. The causes of
the modifications which characterise classificatory groups are unknown;
the second class of modifications occur in response to adaptational
requirements.
Reichert's two papers are of considerable importance, and Mueller's
remark in his review[209] of them is on the whole justified. "These
praiseworthy investigations supply from the realm of embryology new and
welcome foundations for comparative anatomy" (p. clxxxvii.).
The development of the skull was, however, more thoroughly worked out by
Rathke, and with less theoretical bias, in his classical paper on the
adder.[210] This memoir of Rathke's is an exhaustive one and deals with
the development of all the principal organ-systems, but particularly of
the skeletal and vascular. He confirmed in its essentials Reichert's
account of the metamorphoses of the first two visceral arches,
describing how the rudiment of the skeleton of the first arch appears as
a forked process of the cranial basis, the upper prong developing into
the palatine and pterygoid, the lower forming Meckel's cartilage, while
the quadrate develops from the angle of the fork. The actual bone of the
upper jaw (maxillary) develops outside and separate from the
palato-pterygoid bar. The cartilaginous rod supporting the second
visceral arch divides into three pieces on each side, of which the lower
two form the hyoid, the uppermost the columella. Like Reichert he held
the visceral arches to be parts of the visceral plates, containing,
however, elements from all three germ-layers--the serous, mucous, and
vessel layers.
The first gill-slit, or, as Rathke here prefers to call it, pharyngeal
slit, closes completely in snakes and in Urodeles. It forms the
Eustachian tube in all other Tetrapoda. As regards the vertebrae, Rathke
describes them as being formed in the sheath of the chorda from paired
rudiments, each of which sends two branches upwards, and two branches
downwards. The two inner pairs of processes coalesce round the chorda,
and later form the centrum; the upper outer pair meet above the spinal
column; the lower outer pair form ribs. The odontoid process of the axis
vertebra is the centrum of the atlas (p. 120). The formation of
vertebral rudiments begins close behind the ear-labyrinth, but in front
of this the chorda-sheath gives origin to a flat membranous plate which
afterwards becomes cartilaginous. This plate reaches forward below the
third cerebral vesicle as far as the infundibulum. The notochord ends in
this plate, which is the _basis cranii_, just at the level of the
ear-labyrinth. In no Vertebrate does the notochord extend farther
forward (p. 122). The _basis cranii_ gives off three trabeculae. The
middle one is small and sticks up behind the infundibulum; it is absent
in fish and Amphibia, and soon disappears during the development of the
higher forms. The lateral trabeculae are long bars which curve round the
infundibulum and reach nearly to the front end of the head. Together
they are lyre-shaped. The cranial basis and the trabeculae are formed,
like the vertebrae, in the sheath of the notochord, and the only
differences between the two in the early stage of their development are
that the formative mass for the cranial basis is much greater in amount
than that for the vertebrae, and that the cranial basis by means of its
processes, the trabeculae, reaches well in front of the terminal portion
of the notochord (p. 36). The capsule for the ear-labyrinth develops
quite independently of the cranial basis and the notochord. It resembles
on its first appearance, in form, position, composition, and
connections, the ear-capsule of Cyclostomes, and so do the ear-capsules
of all embryonic Vertebrates (p. 39). It manifests clearly the embryonic
archetype, ... "there exists one single and original plan of formation,
as we may suppose, upon which is built the labyrinth of Vertebrates in
general" (p. 40). When ossification sets in, the ear-capsule forms three
bones, of which two fuse with the supraoccipital and exoccipitals.
[Illustration: FIG. 11.--Embryonic Cranium of the Adder. Ventral Aspect.
(After Rathke.)]
During the formation of the ear-capsule the cranial basis develops from
a plate to a trench, for in its hinder section the side parts grow up to
form the side walls of the brain, in exactly the same way as the
processes of the vertebral rudiments grow up to enclose the spinal
column (pp. 122, 192). The foundations of the skull are now complete,
and ossification gradually sets in. The basioccipital is formed
in the posterior part of the _basis cranii_, and the exoccipitals in the
side walls of the trench in continuity with the fundament of the
basioccipital (see Fig. 11). The supraoccipital is formed in cartilage
above the exoccipitals. The basisphenoid develops, like the
basioccipital, in the flat _basis cranii_, but towards its anterior
edge, between the large foramen (_h_) and the pituitary space (_i_). It
is formed from two centres, each of which is originally a ring round the
carotid foramen. The presphenoid develops in isolation between the
lateral trabeculae, just behind the point where they fuse. The side parts
of the basisphenoid and presphenoid (forming the alisphenoids and the
orbitosphenoids respectively) develop in cartilage separately from the
cranial basis, not like the exoccipitals in continuity with it. The
hinder parts of the trabeculae become enclosed by two processes of the
basisphenoid; their front parts remain in a vestigial and cartilaginous
state alongside the presphenoid. The frontals and parietals show a
peculiar mode of origin in the adder, differing from their origin in
other Vertebrates. The frontals develop in continuity with the
orbitosphenoids, the parietals in continuity with the alisphenoids, and
so have much resemblance with the vertebral neural arches which surround
the spinal column (p. 195).
Through Rathke's work the real embryonic archetype of the vertebrate
skull was for the first time disclosed. Rathke discussed this archetype
and its relation to the vertebral theory of the skull in another paper
of the same year (1839), but before going on to this paper, we shall
quote from the paper on the adder the following passage, remarkable for
the clear way in which the idea of the embryological archetype is
expressed. "Whatever differences may appear in the development of
Vertebrates, there yet exists for the different classes and orders a
universally valid idea (plan, schema, or type) ruling the first
formation of their separate parts. This idea must first be worked out,
though possibly with modifications, before more special ideas can find
play. The result of the latter process, however, is that what was formed
by the first idea is not so much hidden as partially or wholly
destroyed" (p. 135).
Rathke's general paper on the development of the skull in Vertebrates[211]
treats the matter on a broader comparative basis than his paper on the
adder, and takes into account all the vertebrate classes, in so far as
their development was then known. He here makes the interesting
suggestion, later entirely confirmed, that the _basis cranii_ or basilar
plate is first laid down as two strips, one on each side of the
chorda--the structures now known as parachordals (pp. 6, 27). For this
supposition, he thinks, speaks the structure of the skull in
_Ammocoetes_, which in this respect is the simplest of all Vertebrates
(pp. 6, 22). In _Ammocoetes_, as Johannes Mueller had shown, the
foundation of the skull is formed by two long cartilaginous bars,
between the hinder portions of which the notochord ends. In these Rathke
was inclined to see the homologues of his trabeculae, and of the
parachordals which he was ready to assume from his embryological
observations.
Mueller was, of course, very ready to accept Rathke's opinions on this
subject, for he considered that they supported his own theory of the
vertebral nature of the skull. After describing in his _Handbuch der
Physiologie_ the cartilaginous bands in _Ammocoetes_ and their highly
differentiated homologues in the Myxinoids, he writes in the later
editions, "Hence we see that in the cranium, as in the spinal column,
there are at first developed at the sides of the chorda dorsalis two
symmetrical elements, which subsequently coalesce, and may wholly
enclose the chorda. Rathke has recently observed, in the embryos of
serpents and other animals, before the formation of the proper cranial
vertebrae, two symmetrical bands of cartilage, similar to those which I
discovered as a persistent structure in _Ammocoetes_.... At a later
period the _basis cranii_ of vertebrate animals contains three parts
analogous to the bodies of vertebrae, the most anterior of which, in the
majority of animals, is generally small, and its development frequently
abortive, whilst in man and mammiferous animals the three are very
distinct. These parts are developed by the formation of three distinct
points of ossification, one behind the other, in the basilar
cartilage."[212]
Rathke was very cautious about accepting the vertebral theory of the
skull; he saw that the facts of development were not altogether
favourable to the theory, and he gave his adherence with many
reservations and saving clauses. His general attitude may be summed up
as follows.[213]
The chorda sheath is the common matrix of the vertebrae and of a large
part of the skull. The basilar plate and the trabeculae, which are
developed from the chorda sheath, give origin to three bones, which
might possibly be considered equivalent to vertebral centra--the
basioccipital, the basisphenoid, and the _Riechbein_ (ethmoid). The
_Riechbein_ develops from the fused ends of the trabeculae. The
presphenoid might also be considered as a vertebral body, but it
develops independently of the basilar plate and trabeculae.
Now of these bones, the basioccipital is in every way equivalent to a
vertebral centrum, for it develops in the basilar plate round the
notochord. With the exoccipitals, which arise just like neural arches,
it forms a true vertebra. The supraoccipital is an accessory bone
developed in relation to bigger brains. The basisphenoid appears in the
basilar plate, but in front of the notochord, nor does it arise in
exactly the same way as the centrum of a vertebra. The basisphenoid with
the alisphenoids, which develop independently in the side walls of the
brain, may, however, still be considered as forming a vertebra, though
the resemblance is not so great as in the case of the occipital ring.
The presphenoid, being long and pointed, is very unlike a vertebral
body. The orbitosphenoids develop separately from it. The ethmoid also
differs from a vertebra, for it surrounds not the whole nervous axis as
the two hinder "vertebrae" do, but only two prolongations of it, the
olfactory lobes. In its development and final form it shows no
particular resemblance to a vertebra. Its body, the _pars
perpendicularis_ (mesethmoid) shows no similarity with a vertebral
centrum. Completing the three hinder cranial "vertebrae" and roofing in
the brain are the supraoccipital, the parietals and the frontals. The
premaxillaries, vomer, and nasals do not belong to the cranial scheme;
they are covering bones connected with the ethmoid. So, too, the
ear-capsule is not part of the cranial vertebrae, but is rather to be
compared to the intercalary bones in the vertebral column of certain
fish. Summing up as regards the cranial vertebrae Rathke writes, "We find
that the four different groups of bones, consisting of the basioccipital
with its intercalary (the supraoccipital), the basisphenoid with its
intercalaries (parietals), the presphenoid with its intercalaries
(frontals), and the ethmoid with its outgrowths (turbinals and
cribriform plate), taking them in order from behind forwards, show an
increasing divergence from the plan according to which vertebrae as
commonly understood develop, so that the basioccipital shows the
greatest resemblance to a vertebra, the ethmoid the least" (p. 30).
In a posthumous volume published in 1861 the same opinion is put
forward. "In the head, too," he writes, "some vertebrae can be
recognised, although in a more or less modified form. Yet at most only
four cranial vertebrae can be assumed, and these differ from ordinary
well-developed vertebrae in their manner of formation the more the
farther forward they lie."[214]
Rathke was an able and careful critic of the vertebral theory of the
skull, but he accepted it in the main. Actual attack on the theory upon
embryological grounds was begun by C. Vogt, in his work on the
development of _Coregonus_,[215] and in his paper on the development of
_Alytes_.[216] He described for _Coregonus_ an origin of the skull in the
main similar to that established by Rathke for the adder. There was a
"nuchal plate" in which the front end of the notochord was imbedded; the
notochord ended at the level of the labyrinth; there were two lateral
bands, comparable to Rathke's lateral trabeculae; a "facial plate" was
also formed, which seems on the whole equivalent to the plate formed by
the fused anterior ends of the trabeculae. A little later the cranium
formed a complete cartilaginous box surrounding the brain, very similar
to the adult cranium of a shark.
In his criticism of the vertebral theory of the skull, Vogt started by
defining the vertebra as a ring formed round the chorda. Now since only
the occipital segment of the skull is formed actually round the
notochord, the parts of the skull lying in front of this cannot
themselves be vertebrae, though they may be considered as prolongations
of the occipital or nuchal vertebra. "We must regard the nuchal plate as
a true vertebra, modified, it is true, in its formation and development
by its particular functions. Now, since the notochord ends with the
nuchal plate we can no longer regard as vertebrae the parts of the skull
that lie beyond, such as the lateral processes of the cranium and the
facial plate, for they have no relation with the notochord" (p. 123).
To support this view he adduced the fact that the vertebral divisions
(primitive vertebrae) visible in the trunk do not extend into the head.
He used precisely the same arguments in his paper on _Alytes_ to destroy
the vertebral theory of the skull. We quote the following passage
translated by Huxley (1864, p. 295) from this paper. "It has therefore
become my distinct persuasion that the occipital vertebra is indeed a
true vertebra, but that everything which lies before it is not fashioned
upon the vertebrate type at all, and that efforts to interpret it in
such a way are vain; that, therefore, if we except that vertebra
(occipital) which ends the spinal column anteriorly, there are no
cranial vertebrae at all."
L. Agassiz, himself a pupil of Doellinger, in the general part (1844) of
his _Recherches sur les Poissons fossiles_ (Neuchatel, 1833-43), repeats
in the main his pupil Vogt's criticism of the vertebral theory (vol. i.,
pp. 125-9).
These arguments of Vogt and Agassiz were not considered by Mueller to
dispose of the theory,[217] which maintained a firm hold even upon
embryologists. It was still upheld by Reichert, and Koelliker in 1849
showed himself convinced of its general validity.
A useful step in the analysis of the concept "vertebra" was taken by
Remak,[218] who showed what a complex affair the formation of vertebrae
really is, involving as it does a complete resegmentation
(_Neugliederung_) of the vertebral column, whereby the original
vertebral bodies were replaced by the secondary definitive bodies (p.
143). Remak showed, as he thought, that the protovertebral segmentation
of the dorsal muscle-plates did not extend into the head, and he denied
Reichert's assertion (1837) that the cranial basis in mammals showed
transverse grooves delimiting three cranial vertebrae (p. 36). The
gill-slits, he considered, could not possibly be regarded as marking the
limits of head vertebrae.
In 1858 appeared Huxley's well-known Croonian Lecture, _On the Theory of
the Vertebrate Skull_,[219] in which he stated with great clearness and
force the case for the embryological method of determining homologies,
and criticised with vigour the vertebral theory of the skull. By this
time the two rival methods in morphology had become clearly
differentiated, and Huxley was able to contrast them, or at least to
show how necessary the new embryological method was as a corrective and
a supplement to the older anatomical, or, as he calls it, "gradation"
method. Applied to the "Theory of the Skull," the gradation method
consists in comparing the parts of the skull and vertebral column in
adult animals with respect to their form and connections. "Using the
other method, the investigator traces back skull and vertebral column to
their earliest embryonic states and determines the identity of parts by
their developmental relations" (p. 541). This second method is the final
and ultimate. "The study of the gradations of structure presented by a
series of living beings may have the utmost value in suggesting
homologies, but the study of development alone can finally demonstrate
them" (p. 541). As an example of the utility and, indeed, the necessity
of applying the embryological method Huxley takes the case of the
quadrate bone in birds. This bone had been generally regarded by
anatomists as the equivalent of the tympanic of mammals, on account of
its connection with the tympanum; but Reichert showed (1837) that the
same segment of the first visceral arch developed into the incus in
mammals, and into the quadrate in birds, and that therefore the quadrate
was homologous with the incus. Similarly, on developmental grounds, the
malleus or hammer of mammals is the homologue of the articular of birds,
since both are developed from a portion of Meckel's cartilage identical
in form and connections in the two groups. The homologies of the bones
connected with the jaws in bony fishes had long been a subject of
contention among comparative anatomists; Huxley shows from his personal
observations how the development of the visceral arches throws light
upon these difficulties. The mandibular arch in the developing fish is
abruptly angled, as in the embryo of Tetrapoda; the upper prong of it
ossifies into the palatine and pterygoid; at the angle is formed the
quadrate (jugal, Cuvier), and to the quadrate is articulated the lower
jaw, which ossifies round the lower prong or Meckel's cartilage. The
scheme of development of the jaws is accordingly similar in fish to what
it is in other Vertebrates, and this similarity of development enables
Huxley to recognise what are the true homologues of the quadrate, the
palatine and the pterygoid in adult bony fish, and to prove that the
symplectic and the metapterygoid (tympanal, Cuvier) are bones peculiar
to fish. In developing Amphibia Huxley found a suspensorium of hyoid and
mandibular arches similar to the hyomandibular of fish.
Tackling his main problem of the unity of plan of the vertebrate skull,
Huxley shows, by a careful discussion of the anatomical relationships of
the chief bones in typical examples of all vertebrate classes, that
there is on the whole unity of plan as regards the osseous skull. This
unity of composition can be established, on the gradation method, by
considering the connections of the bones of the skull with one another,
their relations to the parts of the brain and to the foramina of the
principal cranial nerves. The assistance of the embryological method is,
however, necessary in determining many points with regard to the bones
developed in relation to the visceral arches. But there is a further
step to be taken. "Admitting ... that a general unity of plan pervades
the organisation of the ossified skull, the important fact remains that
many vertebrated animals--all those fishes, in fact, which are known as
_Elasmobranchii_, _Marsipobranchii_, _Pharyngobranchii_ and _Dipnoi_
have no bony skull at all, at least in the sense in which the words have
hitherto been used" (p. 571). The membranous or cartilaginous skull of
these fishes shows a general resemblance in its main features to the
ossified skull of other Vertebrates; the relations of the ear to the
vagus and trigeminal nerves are, for instance, the same in both; the
main regions of the cartilaginous skull can be homologised with definite
bones or groups of bones in the bony skull; but discrepancies occur. It
is again to development that we must turn to discover the true
relationship of the cartilaginous to the ossified skull. "The study of
the development of the ossified vertebrate skull ... satisfactorily
proves that the adult crania of the lower _Vertebrata_ are but special
developments[220] of conditions through which the embryonic crania of
the highest members of the sub-kingdom pass" (p. 573). It is with the
embryonic cranium of higher Vertebrates that the adult skull of the
lower fishes must be compared, and the comparison will show a
substantial though not a complete agreement between them. Thus, speaking
of the development of the frog's skull, Huxley writes:--"If, bearing in
mind the changes which are undergone by the palatosuspensorial
apparatus, ... we now compare the stages of development of the frog's
skull with the persistent conditions of the skull in the _Amphioxus_,
the lamprey, and the shark, we shall discover the model and type of the
latter in the former. The skull of the _Amphioxus_ presents a
modification of that plan which is exhibited by the frog's skull when
its walls are still membranous and the notochord is not yet embedded in
cartilage. The skull of the lamprey is readily reducible to the same
plan of structure as that which is exhibited by the tadpole when its
gills are still external and its blood colourless. And finally, the
skull of the shark is at once intelligible when we have studied the
cranium in further advanced larvae, or its cartilaginous basis in the
adult frog" (p. 577). Development, therefore, proves what comparative
anatomy could only foreshadow--the unity of plan of all vertebrate
skulls, ossified and unossified alike. "We have thus attained to a
theory or general expression of the laws of structure of the skull. All
vertebrate skulls are originally alike; in all (save _Amphioxus_?) the
base of the primitive cranium undergoes the mesocephalic flexure, behind
which the notochord terminates, while immediately in front of it the
pituitary body is developed;[221] in all, the cartilaginous cranium has
primarily the same structure--a basal plate enveloping the end of the
notochord and sending forth three processes, of which one is short and
median, while the other two, the lateral trabeculae, pass on each side of
the space on which the pituitary body rests, and unite in front of it;
in all, the mandibular arch is primarily attached behind the level of
the pituitary space, and the auditory capsules are enveloped by a
cartilaginous mass, continuous with the basal plate between them. The
amount of further development to which the primary skull may attain
varies, and no distinct ossifications at all may take place in it; but
when such ossification does occur, the same bones are developed in
similar relations to the primitive cartilaginous skull" (p. 578).
In a word, there is a general plan or primordial type which is
manifested in the higher forms most clearly in their earliest
development--an embryological archetype therefore.
Huxley now goes on to consider the relation of this general plan or type
of the skull to the structure and development of the vertebral column.
Does the skull in its development show any signs of a composition out of
several vertebrae? The vertebral column develops as a segmented structure
round the notochord; the skull develops first as an unsegmented plate
extending far beyond the notochord. The processes of this basilar plate,
the trabeculae, are quite unlike anything in the vertebral column. It is
true that when the process of ossification begins, separate bones are
differentiated in the basilar plate one in front of the other, giving an
appearance of segmentation. The hindmost of these bones, the
basioccipital, ossifies round the notochord, quite like a vertebral
centrum, and its side parts which form the occipital arch develop in a
"remotely similar" way to the neural arches of the vertebrae. The next
bone, however, the basisphenoid, develops in front of the notochord, and
shows very little analogy with a vertebral body. The analogy is even
more far-fetched when applied to the axial bones in front of the
basisphenoid. The cranium might indeed be divided upon ossification into
a series of segments bearing a more or less remote analogy with
vertebrae. "In the process of ossification there is a certain analogy
between the spinal column and the cranium, but that analogy becomes
weaker and weaker as we proceed towards the anterior end of the skull"
(p. 585). The best way to state the facts is to say that both skull and
vertebral column start in their development from the same point, but
immediately begin to diverge. The clear indications of segmentation
which fully ossified adult skulls undoubtedly show are, therefore,
secondary, and the vertebral theory of the skull, which was originally
based upon the appearance of such fully ossified crania, is on the whole
negatived by embryology.
We have now to turn back a few years in order to follow up another line
of discovery which had an important bearing upon the theory of the
vertebrate skull--the working out of the distinction between membrane
and cartilage bones.
As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal
College of Surgeons, demonstrated that in the human foetus some bones
were formed not in cartilage but directly in fibrous tissue, and this
observation was confirmed by other human anatomists, particularly by
Sharpey at a considerably later date. In 1822 Arendt[223] focussed
attention upon the remarkable structure of the skull of the Pike, with
its cartilaginous brain-box studded all over with bony plaques, an
arrangement which had already attracted the interest of Cuvier and
Meckel. K. E. von Baer[224] in 1826 discussed at some length the relation
between the bony and the cartilaginous skull in fishes, with particular
reference to the sturgeon, coming to the following just conclusion:--"If
we consider the fibrous skeleton of _Ammocoetes_ as the first foundation
of the skeleton of Vertebrates, we can form a series among the
cartilaginous fishes, according as a cartilaginous skeleton penetrates
more and more into this fibrous foundation. In the same way the process
of ossification supplants the cartilaginous skeleton. So long as the
ossifications lie in the skin, as in the sturgeon, they form corneous
bones (_Hornknochen_), but when they lie under the skin, they form true
bones, _e.g._, the bones of the skull in the pike" (p. 374).
Embryologists soon become aware that a similar distinction between a
primitive cartilaginous foundation and a secondary overlying
ossification of the skull showed itself in the development of all
Vertebrates. Duges, in his _Recherches sur l'osteologie et la myologie
des Batraciens_ (1834), distinguished between such bones as are formed
by direct ossification of the cartilaginous groundwork of the skull, and
such as are developed in the periosteal fibrous tissue.
Reichert in 1838[225] noted that several of the skull bones in Amphibia
are formed without the intermediary of cartilage, such as the nasals,
the maxillaries and the lacrymals. So, too, the frontals and parietals
of Teleosts developed independently of the cartilaginous skull, and
belonged to the skeletal system of the skin, not to the true vertebral
axial skeleton (pp. 215-6). Even more interesting was his discovery,
afterwards confirmed by Hertwig,[226] that in the newt several bones
connected with the palate were formed in the mucous membrane of the
mouth by the fusion of a number of little conical teeth (p. 97). Certain
of these bones he considered to be the substitutes, not the equivalents,
of the palatine and pterygoid of other Vertebrates, which are formed
from the upper part of the first visceral arch, a part missing in the
newt (p. 100). Owing to the difference of development he would not
homologise these bones in the newt with the palatine and pterygoid of
other Vertebrates. He recognised also that the bone now known as the
parasphenoid was developed in the frog in the mucous membrane of the
mouth, and had originally no connection with the cranial basis (p. 34).
Rathke in 1839 also allowed the distinction between cartilage and
membrane bone, but laid no stress upon it (_Entw. d. Natter._, p. 197).
Jacobson in 1842[227] introduced the useful term, "primordial cranium,"
for the primitive cartilaginous foundation of the skull, and drew a
sharp distinction between cartilage bones and membrane bones.
In his _Recherches sur les Poissons fossiles_,[228] L. Agassiz used Vogt's
work on the development of _Coregonus_ to establish a classification of
the bones of the skull in fish, a classification which had the merit of
drawing a sharp distinction between the cartilaginous groundwork and
the "protective plates" of the fish's skull. He recognised that the
protective plates developed in a different way from the other bones of
the skull. "We must distinguish," he writes, "two kinds of ossification;
one which tends to transform the primitive parts of the embryonic
cranium directly into bone, and another which leads to the deposition of
protective plates round this core, which develop not only upon the upper
surface, as has hitherto been supposed, but also on the lateral walls
and on the lower surface of the cranium" (p. 112). In the skull of all
fish there are three elements--(1) the cartilaginous base, including the
nuchal plate, the trabeculae and the facial plate, together with the
auditory capsules; (2) the cartilaginous cerebral envelope; (3) the bony
protective plates (absent in Elasmobranchs). The bones developed in
relation to these cranial elements can be classified as follows:--(1)
the basioccipital, exoccipitals (paroccipitals?), supraoccipital and
"petrous" (_rocher_), developed from the nuchal plate; the ali- and
orbito-sphenoids developed from the trabeculae; the "cranial ethmoid"[229]
developed from the facial plate; (2) the parietals, frontals and nasals
formed from the "superior" protective plate; the "anterior" and
"posterior" frontals and the temporal, from the "lateral" plates; the
body of the sphenoid and the vomer from the "inferior" plates. The other
element, the cartilaginous brain-box, does not ossify, and tends to
become absorbed (p. 124).
In 1849 Koelliker published a paper[230] dealing with the morphological
significance of the distinction between membrane and cartilage bones,
and in 1850[231] he defended his views against the criticisms of
Reichert[232] in a further note entitled _Die Theorie des
Primordialschaedels festgehalten_. It is convenient to consider these
papers together. Koelliker held that there was (1) a histological and (2)
a morphological difference between the two categories of bones. The
histological development of the two kinds was different, but this
difference was not sufficient to establish a morphological distinction
between them, a distinction in their anatomical _Bedeutung_. The true
morphological distinction between them was their development in
different skeleton-forming layers. Membrane bones were developed in
fibrous tissue lying between the skin and the deep layer which formed
the primordial cranium, and it was this formation in a separate layer
that gave them a different morphological significance from the bones
formed directly in the deep layer. Koelliker's distinction, therefore,
was between the bones formed in the primordial cartilaginous cranium on
the one hand, and the superficial ossifications in fibrous tissue on the
other hand. The cartilaginous cranium in Koelliker's opinion was formed
upon the vertebral type, and the membrane bones were accessory. This, at
least, was his opinion in 1849. In 1850, after Stannius had shown that
membrane bones occurred as integral parts of the vertebrae in certain
fish, he modified his view of the membrane bones, and admitted them, at
least in some cases, as constituents of the cranial vertebrae.
On this morphological distinction of membrane and cartilage bones future
comparative osteology was to be based:--
"My sole aim is to state again the principle upon which comparative
osteology is to be based and extended, and this is that first place
should be assigned to anatomical considerations, and among these to the
manner of origin of the whole bone in relation to the skeleton-forming
layers" (1850, p. 290).
The homologies established by this new principle might run counter to
the homologies indicated by the study of adult structure. "Thus, for
instance, although the lower jaw in position, function, form and shape,
appears to be the same bone throughout, yet it must be admitted that it
shows a difference in the different classes. In Mammals and Man it is an
entirely secondary bone (an extremity according to Reichert), in Birds,
Amphibia and Fishes only partially so, for its articular belongs to
Meckel's cartilage and is accordingly analogous to a rib; indeed, in the
Plagiostomes, etc., the whole lower jaw along with the articular is a
persistent Meckel's cartilage" (p. 290, 1850).
So, too, the supraoccipital in man cannot be fully homologised with the
supraoccipital of many mammals, for its upper half arises at first in
isolation as a secondary bone (p. 290).
Reichert objected to the distinction drawn by Koelliker, and denied that
there was either a histological or a morphological difference between
membrane and cartilage bones. It was shown a few years later by H.
Mueller[233] that there was in truth no essential difference in
histological development between the two categories of bone, that the
cartilage cells were replaced by bone cells identical with those taking
part in the formation of membrane bones. The morphological distinction
continued however to be recognised, particularly by the embryologists.
Rathke in his volume of 1861[234] classified the bones of the skull
according to their origin from the primordial cranium or from the
overlying fibrous layer, distinguishing as membrane bones, the
parietals, frontals, nasals, lachrymals, maxillaries and premaxillaries,
jugals, tympanic, parts of the "temporal," vomer, part of the
supraoccipitals in some mammals, and the mandible (with the exception of
the articular in such as have a quadrate bone). Huxley was also inclined
in 1864[235] to recognise the distinction, but he writes with some
reserve:--"Is there a clear line of demarcation between membrane bones
and cartilage bones? Are certain bones always developed primarily from
cartilage, while certain others as constantly originate in membrane? And
further, if a membrane bone is found in the position ordinarily occupied
by a cartilage bone, is it to be regarded merely as the analogue and not
as the homologue of the latter?" (p. 296).
We may note here that many comparative anatomists of the period were
quite ready to decide Huxley's last question in a sense favourable to
the older, purely anatomical, view of homology. Owen, for instance, held
that difference of development did not disturb homologies established by
form and connections. "Parts are homologous," he writes, "in the sense
in which the term is used in this work, which are not always similarly
developed: thus the 'pars occipitalis stricte dicta,' etc., of
Soemmering is the special homologue of the supraoccipital bone of the
cod, although it is developed out of pre-existing cartilage in the fish
and out of aponeurotic membrane in the human subject."[236] Similarly he
pointed to the diversities of development of the vertebral centrum in
the different vertebrate classes as proof that development could not
always be relied upon in deciding homologies (p. 89). But he could not
deny that the archetype was better shown in the embryo than in the adult
(_supra_, p. 108).
J. V. Carus[237] likewise stood firm for the older method of determining
homologies by comparison of adult structure. "We can regard as
homologous," he writes, "only those parts which in the fully formed
animal possess a like position and show the same topographical relations
to the neighbouring parts" (p. 389). Parts homologous in this sense
might develop in different ways, but no great importance was to be
attached to such a circumstance. Membrane and cartilage bones developed
in practically the same way, from the same skeleton-forming layer, and
no morphological significance attached to their distinction (pp. 227,
457). Embryology was of considerable value in helping to determine
homologies, but the evidence that it supplied was contributory, not
conclusive. Perhaps the greatest service which the study of development
rendered was to disentangle, by a comparison of the earliest embryos,
the generalised type (p. 389).
We have now traced, by our historical study of the theory of the skull,
the gradual evolution of the tendency to find in development the surest
guide to determining homologies. We have seen how the embryological
"type" came to be substituted, in whole or in part, for the anatomical
"type" derived from the study of adult structure. But we have had to do
only with a modification, not with a transformation, of the criterion of
homology recognised by the anatomists. Homology is still determined by
position, by connections, in the embryo as in the adult. "Similarity of
development" has become the criterion of homology in the eyes of the
embryologist, but "similarity of development" means, not identity of
histological differentiation, but similarity of connections throughout
the course of development. For the purposes of morphology, development
has to be considered as an orderly sequence of successive forms, not in
its real nature as a process essentially continuous. Morphology has to
replace the living continuity by a kinematographic succession of stages.
Since it is the earliest of these stages that manifest the simplest and
most generalised structural relations of the parts, it is in the earlier
stages that homologies can be most easily determined. But these
homologies are still determined solely by the relative positions and
connections of the parts, just as homologies are determined in the last
of all the stages of development, the adult state. And since the
generalised type is shown most clearly in the earliest stages and tends
to become obscured by later differentiation, homologies observed in
embryonic life are to be upheld even if the relations in adult life seem
to indicate different interpretations.
[183] See review by Cuvier, _Mem. Mus. Hist, nat._, iii.,
pp. 82-97, 1817.
[184] _Mem. Savans etrangers_, vi. Extract in _Ann. Sci.
nat._ (2) i. (_Zool._), pp. 366-72, 1834.
[185] _Recherches sur la generation des Mammiferes_, 1834.
_Embryogenie comparee_, 1837.
[186] "Kiemen bey Saeugthieren," _Isis_, pp. 747-9, 1825.
[187] "Kiemen bey Voegeln," _Isis_, pp. 1100-1, 1825.
[188] "Ueber die Kiemenbogen und Kiemengefaesse beym
bebrueteten Huehnchen," _Isis_, xx., pp. 401-3, 1827.
(Read in Sept. 1826 to the _Versammlung der deutschen
Naturforscher und Aerzte_, then recently founded by
Oken).
[189] _Isis_, pp. 160-4, Pl. II., 1828.
[190] "Ueber die Kiemen und Kiemengefaesse in den Embryonen
der Wirbelthiere," Meckel's _Archiv_ for 1827, pp.
556-68. Also in _Ann. Sci. nat._, xv., pp. 266-80,
280-4, 1828.
[191] Meckel's _Archiv_, vi., pp. 1-47, 1832.
[192] _Untersuchungen ueber die Bildung und Entwickelung
der Fluss-Krebses_, Leipzig, folio, 1829. Preliminary
notice in _Isis_, pp. 1093-1100, 1825.
[193] "Untersuchungen ueber die Bildung und Entwickelung
der Wasser-Assel.," _Abh. z. Bild. u. Entwick.-Gesch._,
i., pp. 1-20, 1832. Translated in _Ann. Sci. nat._ (2),
ii., (_Zool._), pp. 139-57, 1834.
[194] Koelliker, _Entwickelungsgeschichte_, 2nd ed., p. 17,
Leipzig, 1879.
[195] _Handbuch der Entwickelungsgeschichte des Menschen
und ... der Saeugethiere und Voegel_, Berlin, 1835.
[196] _Embryogenie comparee_, 1837; _Histoire generale du
developpement des corps organises_, 1847-49.
[197] _Entwickelungsgeschichte des Kaninchen-Eies_,
Braunschweig, 1842; _Entwickelungsgeschichte des
Hunde-Eies_, Braunschweig, 1845;
_Entwickelungsgeschichte des Meerschweinchens_, Giessen,
1852; _Entwickelungsgeschichte des Rehes_, Giessen,
1854.
[198] "It is the role of embryology, as my great teacher
says, to form the court of appeal for comparative
anatomy, and it is from embryology particularly, which
has in the last decades provided such signal instances
of the unravelling of obscure problems, that we have to
expect a definite clearing up of the problems relating
to the development of the head."--Mueller's _Archiv_, p.
121, 1837.
[199] _Anat.-phil. Unters. ue. d. Kiemenapparat u. d. Zungenbein_, Riga
and Dorpat, 1832.
[200] "Bildungs- und Entwickelungs-geschichte des Blennius viviparus,"
_Abhandl. z. Bild. u. Entwick.-Gesch. des Menschen u. der Thiere_,
ii., pp. 1-68, Leipzig, 1833.
[201] _Von den Ur-Theilen des Knochen und
Schalen-Gerustes_, Leipzig, 1828.
[202] _Kiemenapparat_, pp. 107-118.
[203] _Vergleichende Anatomie der Myxinoiden_. Part I.
(Osteology and Myology). (_Abh. koenigl. Akad. Wiss.
Berlin_, for 1834, pp. 65-340, 9 pls., 1836.) Also
separately.
[204] "Ueber die Visceralbogen der Wirbelthiere in
Allgemeinen und deren Metamorphosen bei den Voegeln und
Saeugethiere," Mueller's _Archiv_, pp. 120-222, 1837.
[205] _Handbuch d. menschl. Anatomie_, iv., p. 47.
[206] This was shown by Serres (_Ann. Sci. nat._, xi., p.
54 f.n., 1827), who found in a human embryo a long
cartilaginous piece extending from the ear-ossicles to
the inside of the lower jaw, and suggested that it was
the foundation of the permanent mandible.
[207] _Abhandl._, i., p. 102, 1832; ii., p. 25, 1833. (_Blennius_
paper).
[208] _Vergleichende Entwickelungsgeschichte des Kopfes der nackten
Amphibien_, Koenigsberg, quarto, 276 pp., 1838.
[209] Mueller's _Archiv_ for 1838.
[210] _Entwickelungsgeschichte der Natter_, Koenigsberg,
1839.
[211] _Bemerkungen ueber die Entwickelung des Schaedels der
Wirbelthiere_, Koenigsberg, 1839.
[212] _Handbuch der Physiologie des Menschen_, Koblenz,
1835; Eng. trans. by W. Baly, ii., p. 1615, 1838.
[213] For a full statement of Rathke's conclusions, see
the translation given by Huxley in _Lectures on the
Elements of Comparative Anatomy_, London, 1864.
[214] _Entwickelungsgeschichte der Wirbelthiere_, p. 142,
1861.
[215] _Embryologie des Salmones_. A separate volume of L.
Agassiz's _Histoire naturelle des Poissons d'Eau douce
de l'Europe centrale_, Neuchatel, 1842.
[216] _Untersuchungen ueber die Entwickelungsgeschichte der
Gebuertshelferkroete_, Solothurn, 1842.
[217] Mueller's _Archiv_ for 1843, p. ccxlviii.
[218] _Untersuchtingen ueber die Entwickelung der
Wirbelthiere_, Berlin, 1850-55.
[219] Delivered 17th June 1858. Reprinted in _The
Scientific Memoirs of T. H. Huxley_, edited by M. Foster
and E. Ray Lankester, vol. i., pp. 538-606 (1898).
[220] _Cf._ Reichert, _supra_, p. 149.
[221] The origin of the pituitary body from the roof of
the mouth was first described by Rathke (1839).
[222] _Human Osteogeny explained in two Lectures_, London,
1736.
[223] _De capitis ossei Esocis lucii structura singulari.
Dissert. inaug._ Regiomonti, 1822.
[224] "Ueber das aeussere und innere Skelet," Meckel's
_Archiv_, pp. 327-76, 1826.
[225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p.
186).
[226] _Arch. f. mikr. Anat._, xi., Suppl., 1874.
[227] "Om Primordial-Craniet," _Foerhandlingar Skand.
Naturf. Moele_, Stockholm, 1842.
[228] Vol. I., General part, pub. 1844.
[229] _Entosphenoid_, Owen.
[230] _Zweiter Bericht zootom. Anstalt zu Wuerzburg_, 1849.
[231] _Zeits. f. wiss. Zool._, ii., pp. 281-91.
[232] Mueller's _Archiv_ for 1849, pp. 443-515.
[233] _Zeits. f. wiss Zool._, ix., 1858.
[234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861.
[235] _Lectures on the Elements of Comparative Anatomy_.
[236] _On the Archetype of the Vertebrate Skeleton_, p. 5,
1848.
[237] _System der thierischen Morphologie_, Leipzig, 1853.