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 h.o.m.ologies 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 h.o.m.ology 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 h.o.m.ologue 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 h.o.m.ology 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 h.o.m.ology of the former with the ribs was a.s.serted and the h.o.m.ology 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 ca.n.a.l. 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 a.s.serting the substantial h.o.m.ology 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 h.o.m.ologies 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, Trevira.n.u.s, de Blainville and Geoffroy, to establish by anatomical comparison the h.o.m.ologies 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 h.o.m.ologising 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 h.o.m.ologies 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 cla.s.sical 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 pa.s.ses into the head. I do not hesitate to a.s.sert _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 const.i.tute its only resemblance with the vertebral type" (1835, p.
19).
A convinced supporter of the vertebral theory was Johannes Muller, who, in his cla.s.sical 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 princ.i.p.ally 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 h.o.m.ology 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 Muller, 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, Muller 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. Muller 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 Muller, 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. Muller 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 Muller, 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 cla.s.ses. 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 Muller, 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 a.n.a.logy 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-pa.s.sage, internally into the Eustachian tube, and in the middle a part.i.tion 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 h.o.m.ologue of the stapes in mammals.
The third arch takes a varying share, together with the second, in the formation of the hyoid apparatus.
[Ill.u.s.tration: 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 cla.s.sical theory of their h.o.m.ologies. His views were attacked and in some points rectified, but the main h.o.m.ologies 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 (h.o.m.ologue 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 cla.s.ses. 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 a.s.semblage 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 a.s.sociated 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.
[Ill.u.s.tration: 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 cla.s.sificatory 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--cla.s.s modifications and functional modifications. The causes of the modifications which characterise cla.s.sificatory groups are unknown; the second cla.s.s of modifications occur in response to adaptational requirements.
Reichert"s two papers are of considerable importance, and Muller"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. clx.x.xvii.).
The development of the skull was, however, more thoroughly worked out by Rathke, and with less theoretical bias, in his cla.s.sical paper on the adder.[210] This memoir of Rathke"s is an exhaustive one and deals with the development of all the princ.i.p.al 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 p.r.o.ng 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 ma.s.s 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.
[Ill.u.s.tration: 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 s.p.a.ce (_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 pa.s.sage, 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 cla.s.ses 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 cla.s.ses, 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 Muller 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 h.o.m.ologues of his trabeculae, and of the parachordals which he was ready to a.s.sume from his embryological observations.
Muller 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 h.o.m.ologues 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 a.n.a.logous 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 att.i.tude 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 a.s.sumed, 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 pa.s.sage 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. Aga.s.siz, himself a pupil of Dollinger, 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).