Form and Function Part 12

CHAPTER VIII

TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN

Richard Owen is the epigonos of transcendental morphology; in him its guiding ideas find clear expression, and in his writings are no half-truths struggling for utterance. But he was, though a staunch transcendentalist, an eclectic of the older ideas current in his time; for he picked out what was best in the older systems--Cuvier's teleology, Geoffroy's principle of connections, Oken's idea of the serial repet.i.tion of parts. In particular, he a.s.similated the teaching of Cuvier, the great opponent of the transcendentalists, and reconciled it in part with his own transcendentalism. His main theoretical views are to be found in his volume _On the Archetype and h.o.m.ologies of the Vertebrate Skeleton_ (London, 1848). The master-idea of the book is that the vertebrate skeleton consists of a series of comparable segments, each of which Owen calls a vertebra. His definition of a vertebra is, "one of those segments of the endo-skeleton which const.i.tute the axis of the body, and the protecting ca.n.a.ls of the nervous and vascular trunks"

(p. 81). The parts of a typical vertebra are shown in Fig. 4, which is copied from Owen's Fig. 14.

||| zygapophysis ||| -- neural spine ||| *//^*

diapophysis // -- neurapohysis // o ===== --- ===== / ===== |CENTRUM| O ===== -- peiurapophysis / ===== --- ===== / // parapophysis *v//*

/ ||| zygapophysis ||| -- haemal spine |||

FIG. 4.--Ideal Typical Vertebra. (After Owen.)

In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it, the "vertebra" being that of a bird.

[Ill.u.s.tration: FIG. 5.--Natural Typical Vertebra; Thorax of a Bird.

(After Owen.)]

A segment of sternum is included as the "haemal spine" of the vertebra (_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal rib the "haemapophysis" (_h_); the uncinate process of the vertebral rib is known as the "diverging appendage" (_a_). The whole vertebrate skeleton is composed of a series of vertebrae which show these typical parts. We arrive thus at the conception of an "Archetype" of the vertebrate skeleton, such as is represented in Fig. 6.

The archetype is only a scheme of what is usually constant in the vertebrate skeleton, and both the number and the arrangement of the bones in any real Vertebrate are subject to variation. "It has been abundantly proved," Owen writes, towards the end of his volume, "that the idea of a natural segment (vertebra) of the endoskeleton does not necessarily involve the presence of a particular number of pieces, or even a determinate and unchangeable arrangement of them. The great object of my present labour has been to deduce ... the relative value and constancy of the different vertebral elements, and to trace the kind and extent of their variations within the limits of a plain and obvious maintenance of a typical character" (p. 146).

It goes without saying that Owen considered the skull to be formed of vertebrae--the vertebral theory of the skull was, in his system, a deduction from the vertebral theory of the skeleton. He recognised four cranial vertebrae; the arrangement of them, and the relation of their const.i.tuent bones to the parts of the typical vertebra are shown in the table appearing on page 106. So far as their first three elements are concerned, these vertebrae are practically identical with the vertebrae distinguished in the cla.s.sical vertebral theory of the skull, as enunciated by Oken. A divergence appears with the determination of the other elements of the vertebrae. The upper and lower jaws are a.s.sociated with the nasal and frontal vertebrae respectively, not however as limbs of the head, but as const.i.tuent elements of these vertebrae. In the same way the hyoid apparatus is part and parcel of the parietal vertebra, and the pectoral girdle and fore-limbs part of the occipital vertebra.

[Ill.u.s.tration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After Owen.)]

Cranial Vertebrae.[164] (After Owen, 1848, p. 165.)

+---------------+---------------+----------------+---------------+-------------+ | Vertebrae. | Occipital. | Parietal. | Frontal. | Nasal. | +===============+===============+================+===============+=============+ |Centra. |Basioccipital. |Basisphenoid. |Presphenoid. |Vomer. | +---------------+---------------+----------------+---------------+-------------+ |Neurapophyses. |Exoccipital. |Alisphenoid. |Orbitosphenoid.|Prefrontal. | +---------------+---------------+----------------+---------------+-------------+ |Neural Spines. |Supraoccipital.|Parietal. |Frontal. |Nasal. | +---------------+---------------+----------------+---------------+-------------+ |Parapophyses. |Paroccipital. |Mastoid. |Postfrontal. |None. | +---------------+---------------+----------------+---------------+-------------+ |Pleurapophyses.|Scapular. |Stylohyal. |Tympanic. |Palatal. | +---------------+---------------+----------------+---------------+-------------+ |Haemapophyses. |Coracoid. |Ceratohyal. |Articular. |Maxillary. | +---------------+---------------+----------------+---------------+-------------+ |Haemal Spines. |Episternum. |Basihyal. |Dentary. |Premaxillary.| +---------------+---------------+----------------+---------------+-------------+ | Diverging |Fore-limb or |Branchiostegals.|Operculum. |Pterygoid and| | Appendage. | Fin. | | | Zygoma. | +---------------+---------------+----------------+---------------+-------------+

Owen's reasons for considering the pectoral girdle and the fore-limb part of the occipital vertebra are as follows. In fish the pectoral girdle is slung to the skull by means of the post-temporal bone (supra-scapula, according to Owen) which abuts on the occipital arch. In _Lepidosiren_, whose skeleton resembles the archetype in many ways, the pectoral girdle is likewise attached to the occipital segment.

In most other Vertebrates the pectoral girdle has shifted backwards along the vertebral column, by a "metastasis" (Geoffroy) similar to that by which the pelvic fins in many fish have shifted up close to the pectoral girdle. The scapula (with supra-scapula) is the pleurapophysis, the coracoid the haemapophysis, of the occipital vertebra. The clavicle is h.o.m.ologised with the slender bone in fish now known as the post-clavicle, which shows a connection with the first or atlas vertebra of the vertebral column, forming, according to Owen, the haemapophysis of the atlas. Owen considers it no objection to this view that in other Vertebrates the clavicle is anterior to the coracoid--"its anterior position to the coracoid in the air-breathing Vertebrata is no valid argument against the determination, since in these we have shown that the true scapular arch is displaced backwards" (_On the Nature of Limbs_, p. 63, London, 1849). In the pelvic girdle the ilium corresponds to the scapula, the ischium to the coracoid, the pubis to the clavicle.

Hence the ilium is a pleurapophysis, the ischium and pubis are both haemapophyses. The fore-limb is the developed "appendage" of the occipital vertebra, the hind-limb the developed "appendage" of the pelvic vertebra. They are serially h.o.m.ologous with, for example, the uncinate processes of the ribs in birds (see Figs. 5 and 6). The fore-limb is a simple filament in _Lepidosiren_, and presents few joints in _Proteus_ and _Amphiuma_; in other air-breathing Vertebrates it shows a more complete development, the humerus, radius and ulna, and the bones of the wrist and hand becoming differentiated out.

As the fore-limb is equivalent to a single bone of the archetype, it is said to be, in its developed state, "teleologically compound" (p. 103).

Since in the archetype every vertebra has its appendage, more than two pairs of locomotory limbs might have been developed. "Any given appendage might have been the seat of such developments as convert that of the pelvic arch into a locomotive limb; and the true insight into the general h.o.m.ology of limbs leads us to recognise many potential pairs in the typical endoskeleton. The possible and conceivable modifications of the vertebrate archetype are far from having been exhausted in the forms which have hitherto been recognised, from the primaeval fishes of the palaeozoic ocean of this planet up to the present time" (p. 102). It is not of the essence of the vertebrate type to be tetrapodal.

In determining h.o.m.ologies Owen remained true to Geoffroy's principle of connections. Speaking of an attempt which had been made to determine h.o.m.ologies by the mode of development, he writes, "There exists doubtless a close general resemblance in the mode of development of h.o.m.ologous parts; but this is subject to modification, like the forms, proportions, functions, and very substance of such parts, without their essential h.o.m.ological relationships being thereby obliterated. These relationships are mainly, if not wholly, determined by the relative position and connection of the parts, and may exist independently of form, proportions, substance, function and similarity of development.

But the connections must be sought for at every period of development, and the changes of relative position, if any, during growth, must be compared with the connections which the part presents in the cla.s.ses where vegetative repet.i.tion is greatest and adaptive modification least"

(p. 6). It is interesting to note that in Owen's opinion comparative anatomy explains embryology. Thus the scapula, which is the pleurapophysis of the occipital vertebra, is vertical on its first appearance in the embryo of tetrapoda, and lies close up to the head (_On the Nature of Limbs_, p. 49)--the embryo shows a greater resemblance to the archetype than the adult. "We perceive a return to it, as it were, in the early phases of development of the highest organised of the actually existing species, or we ought rather to say that development starts from the old point; and thus, in regard to the scapula, we can explain the constancy of its first appearance close to the head, whether in the human embryo or in that of the swan, also its vertical position to the axis of the spinal column, by its general h.o.m.ology as the rib or 'pleurapophysis' of the occipital vertebra" (_Limbs_, p. 56).

We owe to Owen the first clear distinction between "h.o.m.ologous" and "a.n.a.logous" organs; it was he who first proposed the terms "h.o.m.ologue"

and "a.n.a.logue," which he defined as follows:--"_a.n.a.logue_. A part or organ in one animal which has the same function as another part or organ in a different animal." "_h.o.m.ologue_. The same organ in different animals under every variety of form and function."[165]

He introduced also useful distinctions between Special, General, and Serial h.o.m.ology. "The relations of h.o.m.ology," he writes, "are of three kinds: the first is that above defined, viz., the correspondency of a part or organ, determined by its relative position and connections, with a part or organ in a different animal; the determination of which h.o.m.ology indicates that such animals are constructed on a common type; when, for example, the correspondence of the basilar process of the human occipital bone with the distinct bone called 'basi-occipital' in a fish or crocodile is shown, the _special h.o.m.ology_ of that process is determined. A higher relation of h.o.m.ology is that in which a part or series of parts stands to the fundamental or general type, and its enunciation involves and implies a knowledge of the type on which a natural group of animals, the Vertebrate, for example, is constructed.

Thus when the basilar process of the human occipital bone is determined to be the 'centrum' or 'body' of the last cranial vertebra, its _general h.o.m.ology_ is enunciated.

"If it be admitted that the general type of the vertebrate endoskeleton is rightly represented by the idea of a series of essentially similar segments succeeding each other longitudinally from one end of the body to the other, such segments being for the most part composed of pieces similar in number and arrangement, and though sometimes extremely modified for special functions, yet never so as to wholly mask their typical character--then any given part of one segment may be repeated in the rest of the series, just as one bone may be reproduced in the skeletons of different species, and this kind of repet.i.tion or representative relation in the segments of the same skeleton I call 'serial h.o.m.ology'" (p. 7). As an example of serial h.o.m.ology we might take the centra of the vertebrae--the vomer, the presphenoid, the basisphenoid, the basioccipital and the series of centra in the spinal column. Such serially repeated parts are called _h.o.m.otypes_ (p. 8).

Not all the bones of the vertebrate skeleton are included in the archetype as const.i.tuents of the vertebrae. Thus the branchial and pharyngeal arches are accounted part of the splanchnoskeleton, as belonging to the same category as the heart bone of some ruminants, and the ossicles of the stomach in the lobster (p. 70). The ossicles of the ear in mammals are "peculiar mammalian productions in relation to the exalted functions of a special organ of sense" (p. 140, f.n.). This recognition of a possible development of new organs to meet new functions shows unmistakably the influence of Cuvier. Owen was indeed well aware of the importance of the functional aspect of living things, and he often adopted the teleological point of view. As a true morphologist, however, he held that the principle of adaptation does not suffice to explain the existence of special h.o.m.ologies. The ossification of the bones of the skull from separate centres may be purposive in Eutheria, in that it prevents injury to the skull at birth; but how explain on teleological principles the similar ossification from separate centres in marsupials, birds and reptiles? How explain above all the fact that the centres are the same in number and relative position in all these groups? Surely we must accept the idea of an archetype "on which it has pleased the divine Architect to build up certain of his diversified living works" (p. 73).

In his study of centres of ossification, Owen made in point of theory a distinct advance on his predecessors. We saw that Geoffroy recognised the importance of studying the ossification of the skeleton, and that Cuvier accepted such embryological evidence as an aid in determining h.o.m.ologies. Owen pointed out that it was necessary to distinguish between centres of ossification which were teleological in import and such as were purely indicative of h.o.m.ological relationships. Many bones, single in the adult, arise from separate centres of ossification, but we must distinguish between "those centres of ossification that have h.o.m.ological relations, and those that have only teleological ones; _i.e._, between the separate points of ossification of a human bone which typify vertebral elements, often permanently distinct bones in the lower animals; and the separate points which, without such signification, facilitate the progress of osteogeny, and have for their obvious final cause the well-being of the growing animal" (p. 105).

There is, for example, a teleological reason why in mammals and leaping Amphibia (_e.g._, frogs), the long bones should ossify first at their ends, for the brain is thus protected from concussion; in reptiles that creep there is less danger of concussion, and the long bones ossify in the middle (p. 105). But there is no teleological reason why the coracoid process of the scapula should in all mammals develop from a separate centre. The coracoid is however a real vertebral element (haemapophysis), and in monotremes, birds and reptiles it is in the adult a large and separate bone. Its ossification from a separate centre in mammals has therefore a h.o.m.ological significance. The scapula in mammals is an example of what Owen calls a "h.o.m.ologically compound" bone. All those bones which are formed by a coalescence of parts answering to distinct elements of the typical vertebra are "h.o.m.ologically compound"

(p. 105). On the other hand, "All those bones which represent single vertebral elements are 'teleologically compound' when developed from more than one centre, whether such centres subsequently coalesce, or remain distinct, or even become the subject of individual adaptive modifications, with special joints, muscles, etc., for particular offices" (p. 106). The limb-skeleton, corresponding as it does to a single bone of the archetype, is the typical example of a teleologically compound bone. Owen in his definition of teleological compoundness has combined two kinds of adaptation--(1) temporary adaptation of bones to the exigencies of development, birth and growth (_e.g._, development of long bones from separate centres); (2) definitive adaptation of a skeletal part to the functions which it has to perform (_e.g._, teleological structure of limbs). Such adaptations are, so to speak, grafted on the archetype.

Owen's general views on the nature of living things merit some attention. Organic forms, according to Owen, result from the antagonistic working of two principles, of which one brings about a vegetative repet.i.tion of structure, while the other, a teleological principle, shapes the living thing to its functions. The former principle is ill.u.s.trated in the archetype of the vertebrate skeleton, in the segmentation of the Articulates, in the almost mathematical symmetry of Echinoderms, and the actually crystalline spicules of sponges. It is the same principle which causes repet.i.tion of the forms of crystals in the inorganic world. "The repet.i.tion of similar segments in a vertebral column, and of similar elements in a vertebral segment, is a.n.a.logous to the repet.i.tion of similar crystals as the result of polarising force in the growth of an inorganic body" (p. 171). This "general polarising force" it is which mainly produces the similarity of forms, the repet.i.tion of parts, and generally the signs of the unity of organisation. The adaptive or "special organising force" or [Greek: idea], on the other hand, produces the diversity of organic beings. In every species these two forces are at work, and the extent to which the general polarising or "vegetative-repet.i.tion-force" is subdued by the teleological is an index of the grade of the species.

This view is a.n.a.logous to the Geoffroyan conception that the diversity of form is limited by the unity of plan. Owen thus ranges himself with Geoffroy against Cuvier, who considered that diversity of form is limited only by the principle of the adaptation of parts.

[164] Owen introduced most of the names of bones now current.

[165] _Lectures on Invertebrate Animals_, pp. 374, 379, 1843.

CHAPTER IX

KARL ERNST VON BAER

Von Baer was recognised as the founder of embryology even by his contemporaries. His predecessors, Aristotle,[166] Fabricius,[167]

Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] had made a beginning with the study of development; von Baer, by the thoroughness of his observation and the strength of his a.n.a.lysis, made embryology a science.

It was to one of the German transcendentalists that von Baer owed the impulse to study development. Ignatius Dollinger, Professor in Wurzburg, induced three of his pupils, Pander, d'Alton and von Baer, to devote themselves to embryological research. The development of animals was at this time little known, in spite of recent work by Meckel (1815 and 1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816), by Oken (_loc. cit., supra_, p. 90), and some others.

Pander, with whom apparently Dollinger and d'Alton collaborated, was the first to publish his results;[172] von Baer, who through absence from Wurzburg had for a time dropped his embryological studies, started to work in 1819, after the publication of Pander's treatise, and produced in 1828 the first volume of his master-work, _Ueber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_ (Konigsberg, 1828). The second volume followed in 1837, but dates really from 1834, and was published in an incomplete form. This second volume is intended as an introduction to embryology for the use of doctors and science students. In it von Baer describes in full detail the development of many vertebrate types--chick, tortoise, snake, lizard, frog, fish, several mammals and man, basing his remarks largely upon his personal observations, but taking account also of all contemporary work.

A separate account of the development of a fish (_Cyprinus blicca_) appeared in 1835.[173]

We shall concentrate attention on the first volume. This volume contains the first full and adequate account of the development of the chick, followed by a masterly discussion of the laws of development in general.

When we consider that von Baer worked chiefly with a simple microscope and dissecting needles, the minuteness and accuracy of his observations are astonishing. He described the main facts respecting the development of all the princ.i.p.al organs, and if, through lack of the proper means of observation, he erred in detail, he made up for it by his masterly understanding and profound a.n.a.lysis of the essential nature of development. His account of the development of the chick is a model of what a scientific memoir ought to be; the series of "Scholia" which follow contain the deductions he made from the data, and, in so far as they are direct generalisations from experience, they are valid for all time.

The first Scholion is directed against the theory of preformation, and succeeds in refuting it on the ground of simple observation. The theme of the second Scholion is that the essential nature (_die Wesenheit_) of the animal determines its differentiation, that no stage of development is solely determined by the antecedent stage, but that throughout all stages the _Wesenheit_ or idea of the definitive whole exercises guidance. This guidance is shown most clearly in the regulatory processes of the germ, whereby the large individual variations commonly presented by the early embryo are compensated for or neutralised in the course of further development. Baer in this shows himself a vitalist.

It is, however, the third and subsequent Scholia which must here particularly occupy our attention, for it is in these that von Baer comes to grips with morphological problems. Already in the second Scholion he had definitely enunciated the law which runs as a theme throughout the volume, the observational and the theoretical part alike, the law that development is essentially a process of differentiation by which the germ becomes ever more and more individualised. "The essential result of development," he writes, "when we consider it as a whole, is the increasing independence (_Selbstandigkeit_) of the developing animal" (p. 148). In the third Scholion he elaborates this thought and shows that differentiation takes place in triple wise. The three processes of differentiation are "primary differentiation" or layer-formation, "histological differentiation" within the layers, and the "morphological differentiation" of primitive organs.

The first of these differentiations in time is the formation of the germ-layers, which takes place by a splitting or separation of the blastoderm into a series of superimposed lamellae. Baer's account of the process in the chick is as follows:--

"First of all, the germ separates out into heterogeneous layers, which with advancing development acquire ever greater individuality, but even on their first appearance show rudiments of the structures which will characterise them later. Thus in the germ of the bird, so soon as it acquires consistency at the beginning of incubation, we can distinguish an upper smooth continuous surface and a lower more granular surface.

The blastoderm separates thereupon into two distinct layers, of which the lower develops into the plastic body-parts of the embryo, the upper into the animal parts; the lower shows clearly a further division into two closely connected subsidiary layers--the mucous layer and the vessel-layer; the original upper layer also shows a division into two, which form respectively the skin and the parts which I have called the true ventral and dorsal plates--parts which contain in an undifferentiated state the skeletal and muscular systems, the connective tissues, and the nerves belonging to these. In order to have a convenient term for future use, I have named this layer the muscle-layer" (p. 153).

The process of delamination results then in the formation of four layers, of which the upper two (composing the "animal" or "serous"