The Origin of Vertebrates

by Walter Holbrook Gaskell

Lankester and Willey have attempted to give another explanation of the formation of the atrial cavity; they look upon it as originating from a ventral groove, which becomes a canal by the meeting of two outgrowths from the metapleure on each side. This canal then extends dorsalwards on each side, and so forms the atrial cavity; the metapleure still remains in the adult; the somatic muscles in the epipleure of the adult are the original body-muscles, and not extensions into an epipleuric fold, for there is no such fold.

This explanation is a possible conception for the post-branchial portion of the atrium, but is impossible for the branchial region; for, as Macbride points out, as must necessarily be the case, the point of origin of the atrial wall is, in all stages of development, situated at the end of the gill-slit. It shifts in position with the position of the gill-slit, but there can be no backwards extension of the cavity. Macbride therefore agrees with Kowalewsky that the atrial cavity is formed by the simultaneous ventral extension of pleural folds, and of the branchial part of the original pharynx. Thus, in his summing up, he states: "In the larva practically the whole sides and dorsal portion of the pharynx represent merely the hyper-pharyngeal groove and the adjacent epithelium of the pharynx of the adult, the whole of the branchial epithelium of the adult being represented by a very narrow strip of the ventral wall of the pharynx of the larva. The subsequent disproportionate growth of this part of the pharynx of the larva, and of the adjacent portion of the atrial cavity, has given the impression that the atrial cavity grew upwards and displaced other structures, which is not the case."

Further, van Wijhe states that the atrium extends beyond the atriopore right up to the anus, just as must have been the case if the pleural folds originally existed along the whole length of the body. His words are: "Allerdings hat sich das Atrium beim Amphioxus lanceolatus eigenthümlich ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus, d.h. bis an die Wurzel des Schwanzes ausdehnt."

We get, therefore, this conception of the origin of the somatic musculature of the vertebrate. The invertebrate ancestor possessed on each side, along the whole length of its body, a lateral fold or pleuron which was segmented with the body, and capable of movement with the body, because the dorsal longitudinal somatic muscles extended segmentally into each segment of the pleuron. By the ventral extension of these pleural folds, not only was the smooth body-surface of the vertebrate attained, but also the original appendages obliterated as such, leaving only as signs of their existence the branchiæ, the pronephric tubules, and the sense-organs of the lateral line system.

Such an explanation signifies that the somatic trunk-musculature of the vertebrate was derived from the dorsal longitudinal musculature of the body of the arthropod, and not from the ventral longitudinal musculature, and that therefore in the primitive arthropod stage the equivalent of the myotome of the vertebrate did not give origin to the ventral longitudinal muscles of the invertebrate ancestor. Now, as I have said, von Kennel states that in the procœlom of Peripatus a dorsal part (III. in Fig. [157]) is cut off which gives origin to the dorsal body-musculature, while the ventral part which remains (I. and II. in Fig. [157]) gives origin in its appendicular portion (I.) to the muscles of the appendage, and presumably in its ventral somatic portion (II.) to the ventral longitudinal muscles of the body. This dorsal cut-off part might be called the myotome, in the same sense as the corresponding part of the procœlom in the vertebrate is called the myotome. In both cases the muscles derived from it form only a part of the voluntary musculature of the animal, and in both cases the muscles in question are the dorsal longitudinal muscles of the body, to which must be added the dorso-ventral body-muscles. Now, the whole of my theory of the origin of vertebrates arose from the investigation of the structure of the cranial nerves, which led to the conception that their grouping is not, like the spinal, a dual grouping of motor and sensory elements, but a dual grouping to supply two sets of segments, characterized especially by the different embryological origin of their musculature. The one set I called the somatic segmentation, because the muscles belonging to it were the great longitudinal body-muscles; the other I called the splanchnic segmentation, because its muscles were those connected with the branchial and visceral arches. According to my theory, this latter segmentation was due to the segmentation of the appendages in the invertebrate ancestor; and in previous chapters, dealing as they do with the cranial region, attention was especially directed to the way in which the position of the striated splanchnic musculature could be explained by a transformation of the prosomatic and mesosomatic appendages. Now, I am dealing with the metasomatic region, in which it is true the appendages take a very subordinate place, but still something corresponding to the splanchnic segments of the cranial region might fairly be expected to exist, and I therefore desire to emphasize what appears to me to be the fact, that the musculature, which in the region of the trunk would correspond to that derived from the ventral segmentation of the mesoblast in the region of the head, may have arisen not only from the musculature of the appendages, but also from the ventral longitudinal musculature of the body of the invertebrate ancestor, for it seems probable that this latter musculature had nothing to do with the origin of the great longitudinal muscles of the vertebrate body, either dorsal or ventral.

The way in which I imagine the obliteration of the atrial cavity to have taken place is indicated in Fig. [160], B, which is a modification of a section across a trilobite-like animal as represented in Fig. [160], A. As is seen, the pleural folds on each side have nearly met the bulged-out ventral body-surface. A continuation of the same process would give Fig. [160], C, which is, to all intents and purposes, the same as Fig. [159], C, taken from van Wijhe, and shows how the segmental duct is left in the remains of the atrial cavity. The lining walls of the atrial cavity are represented very black, in order to indicate the presence of pigment, as indeed is seen in the corresponding position in Ammocœtes. In these diagrams I have represented the median ventral surface as a large bulged-out bag, without indicating any structures in it except the ventral extension of the procœlom to form the metacœlom. At present I will leave the space between the central nervous system and the ventral mesentery blank, as in the diagrams; in my next chapter I will discuss the possible method of formation within this blank space of the notochord and midgut. Boveri considers that the obliteration of the atrial cavity in the higher vertebrates is not complete, but that its presence is still visible in the shape of the pronephric duct. The evidence of Maas and others that the duct is formed by the fusion of the pronephric tubules is, it seems to me, conclusive against Boveri's view; but yet, as may be seen from my diagrammatic figures, the very place where one would expect to find the last remnant of the atrial cavity is exactly where the pronephric duct is situated. For my own part I should expect to find evidence of a former existence of an atrial cavity rather in the pigment round the pronephros and its duct than in the duct itself.

Fig. 160.—A, Diagram of Section through a Trilobite-like Animal; B, Diagram to illustrate Suggested Obliteration of Appendages and the Formation of an Atrial Cavity by the Ventral Extension of the Pleural Folds; C, Diagram to illustrate the Completion of the Vertebrate Type by the Meeting of the Pleural Folds in the Mid-ventral Line and the Obliteration of the Atrial Cavity.

Al., alimentary canal; N., nervous system; My., myotome; Pl., pleuron; App., appendage; Neph., nephrocœle; Met., metacœle; S.d., segmental duct; At., atrial chamber; V.Mes., ventral mesentery; Mes., mesonephros. The dotted line represents the splanchnopleuric mesoblast in all figures.

The conception that Amphioxus shows us how to account for the great envelope of somatic muscles which wraps round the vertebrate body, in that the ancestor of the vertebrate possessed on each side the body a segmented pleuron, is exactly in accordance with the theory of the origin of vertebrates deduced from the study of Ammocœtes, as already set forth in previous chapters. For we see that one of the striking characteristics of such forms as Bunodes, Hemiaspis, etc., is the presence of segmented pleural flaps on each side of the main part of the body; and if we pass further back to the great group of trilobites, we find in the most manifold form, and in various degrees of extent, the most markedly segmented pleural folds. In fact, the hypothetical figure (Fig. [160], A) which I have deduced from the embryological evidence, might very well represent a cross-section of a trilobite, provided only that each appendage of the trilobite possessed an excretory coxal gland.