Form and Function Part 29

It is very instructive in this connection to note that Dohrn was not, like so many of his contemporaries, a dogmatic materialist, but upheld the commonsense view that vital phenomena must, in the first instance at least, be accepted as they are. "It is for the time being irrelevant,"

he writes, "to squabble over the question as to whether life is a result of physico-chemical processes or an original property (_Urqualitat_) of all being.... Let us take it as given" (p. 75).

Semper's speculations on the genetic affinity of Articulates and Vertebrates are contained in two papers[403] which appeared about the same time as Dohrn's. He openly acknowledges that his work is essentially a continuation of Geoffroy's transcendental speculations, and gives in his second paper a good historical account of the views of his great predecessor. It is a significant fact that evolutionary morphologists very generally held that Geoffroy was right in maintaining against Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in this a strong argument for the monophyletic descent of all animals from one common ancestral form.

In his first paper Semper does little more than break ground; he insists on the fact that both Annelids and Vertebrates are segmented animals, and he points out how close is the a.n.a.logy between the nephridia or "segmental organs" of the former and the excretory (mesonephric) tubules of the latter, upon which he published in the same volume an extensive memoir. At this time he considered _Balanoglossus_--by reason of its gill-slits (its notochord he did not know)--to be the nearest living representative of the ancestral form of Vertebrates and Annelida.

His second paper is a more exhaustive piece of work and deals with every aspect of the problem, both from an anatomical and from an embryological standpoint. It is consciously and admittedly an attempt to apply Geoffroy's principle of the unity of plan and composition to the three great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper follows Geoffroy's lead very closely in maintaining that it is not the position of the organs relative to the ground that must be taken into account in establis.h.i.+ng their h.o.m.ologies, but solely their spatial relations one to another. He holds that dorsum and venter are terms of purely physiological import, and he proposes to subst.i.tute for them the terms neural and cardial (better, haemal) surfaces, either of which may be either dorsal or ventral in position.

Having established this primary principle, Semper has little difficulty in showing that the main organs of the body lie to one another in the same relative positions in Annelida, Arthropoda, and Vertebrata; and this, together with the metameric segmentation common to them all, const.i.tutes his first great argument in favour of their genetic relations.h.i.+p. But he has still to show that Annelids possess at least the rudiments of certain organs which seem to be peculiar to Vertebrates, as the gill-slits, the notochord, and a nervous system developed from the ectoderm of the "dorsal" surface. He takes particular cognisance also of the old distinction drawn by von Baer, that Vertebrates show a "double-symmetrical" mode of development (_evolutio bigemina_), the dorsal muscle-plates forming a tube above the notochord, the ventral plates a tube below the notochord, whereas Articulates do not possess this axis, and form only one tube, namely, that round the "vegetative" organs (_evolutio gemina_). Semper is at pains to prove that _evolutio bigemina_ is characteristic also of Annelidan development.

[Ill.u.s.tration: FIG. 14.--Transverse Section (Inverted) of the Worm _Nais_. (After Semper.)]

He gets his facts from an elaborate study of the process of budding in the _Naidae_, making the somewhat risky a.s.sumption that regeneration takes essentially the same course as embryonic development.

He succeeds in showing--to his own satisfaction at least--that in the formation of new segments in _Nais_ and _Chaetogaster_ a strand of cells appears between the alimentary ca.n.a.l and the nerve-cord, and that from this axial strand the haemal muscle-plates grow out dorsally round the alimentary ca.n.a.l and the neural muscle-plates ventrally round the nerve-cord (see Fig. 14).

This strand of cells, he concludes, must clearly be the notochord, and the type of development is obviously the double-symmetrical met with in Vertebrates.

The nervous system Semper found to develop in the buds of _Nais_ and _Chaetogaster_ by an ectodermal thickening, just as in some Vertebrates.

The cerebral ganglion was formed by the ends of the nerve-cord growing up round the oesophagus and fusing with the paired "sense-plates" which develop from the ectoderm of the head. The cerebral ganglion is accordingly only secondarily haemal in position, and there is no need therefore to seek in Vertebrates for the h.o.m.ologue of the oesophageal commissures of Annelids, as, for instance, Schneider did.

Since the mouth opens on the neural surface in Annelids and on the haemal surface in Vertebrates, Semper considers that they cannot be equivalent structures, and he finds the h.o.m.ologue of the Vertebrate mouth in a little pit on the haemal surface of the head in the leech _Clepsine_ (also in the true mouth of Turbellaria and the proboscis-opening in Nemertines). The primitive Annelid mouth, however, does not appear in the embryogeny of Vertebrates, for the great development of the brain crowds it out of existence.

The h.o.m.ologues of the gill-slits Semper finds in two little ca.n.a.ls in the head of _Chaetogaster_, which open from the pharynx to the exterior.

In Sabellids he describes an elaborate system of gill-ca.n.a.ls, with a supporting cartilaginous framework which forms a real _Kiemenkorb_ or gill-basket, comparable with that of Amphioxus.

Gill-slits, notochord, relation of nervous system, mesonephric tubules, are thus common to Annelids and Vertebrates--what further proof could one desire of the close relations.h.i.+p of these groups? Yet Semper enters into refinements of comparison, seeing, for instance, in the lateral portions of the ventral ganglia (Fig. 14, _sp. g._) the h.o.m.ologues of the spinal ganglia of Vertebrates, and comparing the lateral line of sense organs in Annelids with the lateral line in Anamnia.

He will not admit that Amphioxus and the Ascidians show a closer resemblance to Vertebrates than his beloved Annelids. Amphioxus, he thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids the possession of a notochord, gill-slits, and a "dorsal" nervous system, yet are further removed from Vertebrates than the latter by reason of their lacking that essential characteristic of Vertebrates, metameric segmentation.

Not content with establis.h.i.+ng the unity of plan of Annelids, Arthropods, and Vertebrates, Semper tries to link on the Annelids, as the most primitive group of the three, to the unsegmented worms, and particularly to the Turbellaria. His speculations on this matter may be summed up somewhat as follows:--The common ancestor of all segmented animals is a segmented worm-like form, not quite like any existing type, resembling the Turbellaria in having two nerve strands on the dorsal side and no oesophageal ring, potentially able to develop either the Vertebrate or the Annelid mouth, and so to give origin both to the Articulate and to the Vertebrate series. The common ancestor alike of unsegmented worms and of all segmented types is probably the trochosphere larva, which in the Vertebrates is represented by the simple _Keimblase_ or blastula.

The Annelid theory of Dohrn and Semper was perhaps not so widely accepted as the rival Ascidian theory, but it counted not a few adherents and gave a certain stimulus to comparative morphology. F. M.

Balfour, who pointed out about the same time as Semper the a.n.a.logy between the nephridia of Annelids and the mesonephric tubules of Vertebrates,[405] while not accepting the actual theories of Dohrn and Semper, took up a distinctly favourable att.i.tude to the general idea that Annelids and Vertebrates were descended from a common segmented ancestor. Discussing this question in his cla.s.sical work on the development of Elasmobranch fishes,[406] Balfour came to the conclusion "that we must look for the ancestors of the Chordata, not in allies of the present Chaetopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chaetopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to their existence is well founded, appears now to have perished."[407]

He held that while there was much to be said for the interchange of dorsal and ventral surfaces postulated by Dohrn and Semper, the difficulties involved in the supposition were too great; he preferred, therefore, to a.s.sume that the present Vertebrate mouth was primitive, and not a secondary formation.

His views as to the phylogeny of the Chordata and the genetic relation of the various cla.s.ses to one another are exhibited in the following schema,[408] names of hypothetical groups being printed in capitals, names of degenerate groups in italics:--

Mammalia. Sauropsida.

| | |____________________________| | Proto-Amniota. Amphibia.

| | |_____________________| | Proto-Pentadactyloidei.

| Teleostei. | | | Ganoidei. |____________Dipnoi | | |__________________| | Proto-Ganoidei.

| |____________Holocephali.

| |____________Elasmobranchii.

| Proto-Gnathostomata.

| ____________________| | | _Cyclostomata_. | | | Proto-Vertebrata.

| | | | ____________________|______________________ | | _Cephalochorda_. Protochordata. _Urochorda_.

The hypothetical ancestral forms (Protochordata) possessed a notochord, a ventral suctorial mouth and numerous gill-slits, and were presumably descended from the common ancestor of Annelids and Vertebrates.

Amphioxus and the Ascidians found their place in this schema as degenerate offshoots of the ancestral Protochordates, while the Cyclostomes were in the same way the degenerate modern representatives of the ancestral Protovertebrates.

Balfour's suggestion, that the nervous system in Annelids and Vertebrates might have arisen by the dorsal or ventral coalescence of the lateral nerve cords found in their common ancestor, bore fruit in the speculations of Hubrecht,[409] on the relation of Nemertines to Vertebrates.

The Annelid theory was firmly supported by Eisig, who in his elaborate monograph on the _Capitellidae_[410] maintained against Furbringer the genetic ident.i.ty of the Annelidan nephridia with the kidney tubules of Vertebrates. The independent discovery by E. Meyer[411] and J. T.

Cunningham,[412] of an internal segmental duct in _Lanice_, into which several nephridia opened, seemed to strengthen this view.

Following Ehlers,[413] Eisig found the h.o.m.ologue of the notochord in the accessory intestine of the _Capitellidae_ and _Eunicidae_, which he supposed might easily be transformed, according to the principle of function-change, from a respiratory to a supporting organ. He finally disposed of the alternative notion that the notochord was represented in Annelids by the "giant-fibres" or neurochordal strands which lie close above the nerve-cord, a view held by Kowalevsky,[414] and for a time by Semper. These strands were shown by Eisig, and by Spengel, to be the neurilemmar sheaths of thick nerve fibres which had in many cases degenerated. The view that the content of the neurochordal tubes was nervous in nature was first promulgated by Leydig in 1864.

Much difference of opinion reigned as to the true h.o.m.ologies of the brain and mouth of Annelids and Vertebrates. Beard[415] and others got over the difficulty of the haemal position of the cerebral ganglion in Annelids by supposing that it degenerated and disappeared altogether in the Annelidan ancestor of Vertebrates, and that accordingly it had no h.o.m.ologue in the Vertebrate nervous system. Beard put forward also the ingenious theory that the hypophysis represents the old Annelidan mouth.

Van Beneden and Julin[416] a.s.sumed that in the ancestors of Vertebrates the oesophagus s.h.i.+fted forward between the still unconnected lobes of the brain to open on the haemal surface.

The fundamental a.s.sumption of the Annelid theory, that dorsal and ventral surfaces are morphologically interchangeable, seemed rather bold to many zoologists, and Gegenbaur[417] voiced a common opinion when he rejected as unscientific the comparison of the ventral nerve cord of Articulates with the dorsal nervous system of Vertebrates.

The _Balanoglossus_ theory of Vertebrate descent also belongs, at least in its first form, to the earlier group of evolutionary speculations.

The gill-slits of _Balanoglossus_ were discovered by Kowalevsky as early as 1866.[418] _Tornaria_ was discovered by J. Muller in 1850, but by him considered an Asterid larva; its true nature as the larva of _Balanoglossus_ was made out by Metschnikoff in 1870, who also remarked upon its extraordinary likeness to the larvae of Echinoderms.[419] That it had some relations.h.i.+p with Vertebrates was recognised by Semper, Gegenbaur and others, but the full working-out of its Vertebrate affinities is due to Bateson.[420]

Bateson broke completely with the Dohrn-Semper view that the metamerism of Articulates and Vertebrates must be put down to inheritance from a common ancestor. He held that metamerism was merely a special manifestation of the general property of repet.i.tion, common to all living things (_cf._ Owen's "vegetative force"), and that accordingly "however far back a segmented ancestor of a segmented descendant may possibly be found, yet ultimately the form has still to be sought for in which these repet.i.tions had their origin" (p. 549). The meaning of the phenomenon was obscure, but he was convinced that the explanation was not to be found in ancestry. "This much alone is clear," he wrote, "that the meaning of cases of complex repet.i.tion will not be found in the search for an ancestral form, which, itself presenting this same character, may be twisted into a representation of its supposed descendant. Such forms there may be, but in finding them the real problem is not even resolved a single stage; for from whence was their repet.i.tion derived? The answer to this question can only come in a fuller understanding of the laws of growth and of variation, which are as yet merely terms" (pp. 548-9). It was in following up this line of thought that Bateson produced his monumental _Materials for the Study of Variation_ (1894).

He found a strong positive argument for his theory that Vertebrates are descended from unsegmented forms in the fact that the notochord arises as an unsegmented structure. With the notochord he h.o.m.ologised the supporting rod in the proboscis of _Balanoglossus_, which like the notochord arises from the dorsal wall of the archenteron, and has a vacuolated structure. The gill-slits of _Balanoglossus_, with their close resemblance in detail to those of Amphioxus, Bateson also used as an argument in favour of the phylogenetic relations.h.i.+p of the Enteropneusta and Vertebrata, together with the formation from the ectoderm of a dorsal nerve tube.

Bateson's views attracted considerable attention, and were thought by many to lighten appreciably the obscurity in which the origin of Vertebrates was wrapped. Thus Lankester wrote in his article on Vertebrates[421] in the _Encyclopedia Britannica_:--"It seems that in _Balanoglossus_ we at last find a form which, though no doubt specialised for its burrowing sand-life, and possibly to some extent degenerate, yet has not to any large extent fallen from an ancestral eminence. The ciliated epidermis, the long worm-like form, and the complete absence of segmentation of the body-muscles lead us to forms like the Nemertines. The great proboscis of _Balanoglossus_ may well be compared to the inv.a.g.i.n.able organ similarly placed in the Nemertines.

The collar is the first commencement of a structure destined to a.s.sume great importance in _Cephalochorda_ and _Craniata_, and perhaps protective of a single gill-slit in _Balanoglossus_ before the number of those apertures had been extended. Borrowing, as we may, the nephridia from the Nemertines, and the lateral in addition to the dorsal nerve, we find that _Balanoglossus_ gives the most hopeful hypothetical solution of the pedigree of Vertebrates."

Much doubt was cast upon the Chordate affinities of the Enteropneusta by Spengel in his monograph of the group,[422] but when the development of the coelom came to be more thoroughly worked out in _Balanoglossus_ and Amphioxus, the striking resemblance in this respect between the two forms gave additional support to the Batesonian view.[423]

[386] The stages in the development of microscopical technique are well summarised by R. Burckhardt, _Geschichte der Zoologie_, p. 121, Leipzig 1907.

[387] "Entwickelungsgeschichte des Amphioxus lanceolatus,"

_Mem. Acad. Sci. St Petersbourg_ (Petrograd) (vii.), xi., No. 4, 1867, 17 pp., 3 pls.

[388] "Weitere Studien u. die Entwickelungsgeschichte des Amphioxus lanceolatus," _Arch. fur mikr. Anat._, xiii., pp. 181-204, 1877.

[389] Particularly by Hatschek (1881) and Boveri (1892).

[390] "Entwickelungsgeschichte der einfachen Ascidien,"

_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (vii.), x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien u. die Entwicklung der einfachen Ascidien," _Arch. f. mikr.

Anat._, vii., pp. 101-130, 1871.

[391] _Descent of Man_, i., p. 205, 1871.

[392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872.

[393] _Archives de Biologie_, 1884, 1885, and 1887.