The Origin of Vertebrates

by Walter Holbrook Gaskell

Next comes the question, why was the pronephros not repeated in the meristic repetition that took place during the early vertebrate stage? What, in fact, caused the disappearance of the metasomatic appendages, and the formation of the smooth body-surface of the fish?

The embryological evidence given by van Wijhe and others of the manner in which the original superficially situated pronephros is removed from the surface and caused to assume the deeper position, as seen in the later embryo, is perfectly clear and uniform in all the vertebrate groups. The diagrams at the end of van Wijhe's paper, which I reproduce here, illustrate the process which takes place. At first the myotome (Fig. [159], A) is confined to the dorsal region on each side of the spinal cord and notochord. Then (Fig. [159], B) it separates from the rest of the somite and commences to extend ventrally, thus covering over the pronephros and its duct, until finally (Fig. [159], C) it reaches the mid-ventral line on each side, and the foundations of the great somatic body-muscles are finally laid.

In order, therefore, to understand how the obliteration of the appendages took place, we must first find out what is the past history of the myotomes. Why are they confined at first to the dorsal region of the body, and extend afterwards to the ventral region, forcing by their growth an organ that was originally external in situation to become internal?

In the original discussion at Cambridge, I was accused of violating the important principle that in phylogeny we must look at the most elementary of the animals whose ancestors we seek, and was told that the lowest vertebrate was Amphioxus, not Ammocœtes; that therefore any argument as to the origin of vertebrates must proceed from the consideration of the former and not the latter animal. My reply was then, and is still, that I was considering the cranial region in the first place, and that therefore it was necessary to take the lowest vertebrate which possessed cranial nerves and sense-organs of a distinctly vertebrate character, a criterion evidently not possessed by Amphioxus. Such argument does not apply to the spinal region, so that, now that I have left the cranial region and am considering the spinal, I entirely agree with my critics that Amphioxus is likely to afford valuable help, and ought to be taken into consideration as well as Ammocœtes. The distinction between the value of the spinal (including respiratory) and cranial regions of Amphioxus for drawing phylogenetic conclusions is recognized by Boveri, who says that, in his opinion, "Amphioxus shows simplicity and undifferentiation rather than degeneration. If truly Amphioxus is somewhat degenerated, then it is so in its prehensile and masticatory apparatus, its sense organs, and perhaps its locomotor organs, owing to its method of living."

Fig. 159.—Diagrams to illustrate the Development of the Vertebrate Cœlom. (After van Wijhe.)

N., central nervous system; Nc., notochord; Ao., aorta; Mg., midgut. A, My., myocœle; Mes., mesocœle; Met., metacœle; Hyp., hypomere (pronephric). B and C, My., myotome; Mes., mesonephros; S.d., segmental duct (pronephric); Met., body-cavity.

Hatschek describes in Amphioxus how the cœlom splits into a dorsal segmented portion, the protovertebra, and a ventral unsegmented portion, the lateral plates. He describes in the dorsal part the formation of myotome and sclerotome, as in the Craniota. Also, he describes how the myotome is at first confined to the dorsal region in the neighbourhood of the spinal cord and notochord, and subsequently extends ventrally, until, just as in Ammocœtes, the body is enveloped in a sheet of somatic segmented muscles, the well-known myomeres.

The conclusion to be drawn from this is inevitable. Any explanation of the origin of the somatic muscles in Ammocœtes must also be an explanation of the somatic muscles in Amphioxus, and conversely; so that if in this respect Amphioxus is the more primitive and simpler, then the condition in Ammocœtes must be looked upon as derived from a more primitive condition, similar to that found in Amphioxus. Now, it is well known that a most important distinction exists between Amphioxus and Ammocœtes in the topographical relation of the ventral portion of this muscle-sheet, for in the former it is separated from the gut and the body-cavity by the atrial space, while in the latter there is no such space. Fürbringer therefore concludes, as I have already mentioned, that this space has become obliterated in the Craniota, but that it must be taken into consideration in any attempt at formulating the nature of the ancestors of the vertebrate.

Kowalewsky described this atrial space as formed by the ventral downgrowth of pleural folds on each side of the body, which met in the mid-ventral line and enclosed the branchial portion of the gut. According to this explanation, the whole ventral portion of the somatic musculature of the adult Amphioxus belongs to the extension of the pleural folds, the original body-musculature being confined to the dorsal region. This is expressed roughly on the external surface of Amphioxus by the direction of the connective tissue septa between the myotomes (cf. Fig. [162], B). These septa, as is well known, bend at an angle, the apex of which points towards the head. The part dorsal to the bend represents the part of the muscle belonging to the original body; the part ventral to the bend is the pleural part, and represents the extension into the pleural folds.