These arguments of Vogt and Aga.s.siz were not considered by Muller to dispose of the theory,[217] which maintained a firm hold even upon embryologists. It was still upheld by Reichert, and Kolliker in 1849 showed himself convinced of its general validity.
A useful step in the a.n.a.lysis 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 a.s.sertion (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 h.o.m.ologies, 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 ident.i.ty 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 h.o.m.ologies, 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 h.o.m.ologous with the incus. Similarly, on developmental grounds, the malleus or hammer of mammals is the h.o.m.ologue 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 h.o.m.ologies 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 p.r.o.ng 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 p.r.o.ng 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 h.o.m.ologues of the quadrate, the palatine and the pterygoid in adult bony fish, and to prove that the symplectic and the metapterygoid (tympa.n.a.l, 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 cla.s.ses, 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 princ.i.p.al cranial nerves. The a.s.sistance 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 h.o.m.ologised 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 pa.s.s" (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, pa.s.s on each side of the s.p.a.ce 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 s.p.a.ce, and the auditory capsules are enveloped by a cartilaginous ma.s.s, 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 a.n.a.logy with a vertebral body. The a.n.a.logy 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 a.n.a.logy with vertebrae. "In the process of ossification there is a certain a.n.a.logy between the spinal column and the cranium, but that a.n.a.logy 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 subst.i.tutes, 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 h.o.m.ologise 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. Aga.s.siz used Vogt"s work on the development of _Coregonus_ to establish a cla.s.sification of the bones of the skull in fish, a cla.s.sification 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. .h.i.therto 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 cla.s.sified 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 Kolliker 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 ent.i.tled _Die Theorie des Primordialschadels festgehalten_. It is convenient to consider these papers together. Kolliker 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. Kolliker"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 Kolliker"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 const.i.tuents 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 a.s.signed 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 h.o.m.ologies established by this new principle might run counter to the h.o.m.ologies 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 cla.s.ses. 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 a.n.a.logous 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 h.o.m.ologised 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 Kolliker, 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.
Muller[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] cla.s.sified 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 a.n.a.logue and not as the h.o.m.ologue 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 h.o.m.ology. Owen, for instance, held that difference of development did not disturb h.o.m.ologies established by form and connections. "Parts are h.o.m.ologous," 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 h.o.m.ologue 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 cla.s.ses as proof that development could not always be relied upon in deciding h.o.m.ologies (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 h.o.m.ologies by comparison of adult structure. "We can regard as h.o.m.ologous," 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 h.o.m.ologous in this sense might develop in different ways, but no great importance was to be attached to such a circ.u.mstance. 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 h.o.m.ologies, 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 h.o.m.ologies. We have seen how the embryological "type" came to be subst.i.tuted, 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 h.o.m.ology recognised by the anatomists. h.o.m.ology is still determined by position, by connections, in the embryo as in the adult. "Similarity of development" has become the criterion of h.o.m.ology in the eyes of the embryologist, but "similarity of development" means, not ident.i.ty 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 h.o.m.ologies can be most easily determined. But these h.o.m.ologies are still determined solely by the relative positions and connections of the parts, just as h.o.m.ologies 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, h.o.m.ologies 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 Saugthieren," _Isis_, pp. 747-9, 1825.
[187] "Kiemen bey Vogeln," _Isis_, pp. 1100-1, 1825.
[188] "Ueber die Kiemenbogen und Kiemengefa.s.se beym bebruteten Huhnchen," _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 Kiemengefa.s.se 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 uber die Bildung und Entwickelung der Fluss-Krebses_, Leipzig, folio, 1829. Preliminary notice in _Isis_, pp. 1093-1100, 1825.
[193] "Untersuchungen uber die Bildung und Entwickelung der Wa.s.ser-a.s.sel.," _Abh. z. Bild. u. Entwick.-Gesch._, i., pp. 1-20, 1832. Translated in _Ann. Sci. nat._ (2), ii., (_Zool._), pp. 139-57, 1834.
[194] Kolliker, _Entwickelungsgeschichte_, 2nd ed., p. 17, Leipzig, 1879.
[195] _Handbuch der Entwickelungsgeschichte des Menschen und ... der Saugethiere und Vogel_, 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."--Muller"s _Archiv_, p.
121, 1837.
[199] _Anat.-phil. Unters. u. 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.