The Origin of Vertebrates

by Walter Holbrook Gaskell

In order to make the embryology of these excretory organs quite clear, I will make use of van Wijhe's phraseology and also of his illustrations. He terms the whole cœlomic cavity the procœlom, which is divisible into a ventral unsegmented part, the body-cavity or metacœlom, and a dorsal segmented part, the somite. This latter part again is divided into a dorsal part—the epimere—and a part connecting the dorsal part with the body-cavity, to which therefore he gives the name of mesomere.

The cavity of the epimere disappears, and its walls form the muscle and cutis plates of the body. The part which forms the muscles is known as the myotome, which separates off from the mesomere, leaving the latter as a blind sac—the mesocœlom—communicating by a narrow passage with the body cavity or metacœlom. At the same time, from the mesomere is formed the sclerotome, which gives rise to the skeletal tissues of the vertebræ, etc., so that van Wijhe's epimere and mesomere together correspond to the original term, protovertebra, or somite of Balfour; and when the myotome and sclerotome have separated off, there is still left the intermediate cell-mass of Balfour and Sedgwick, i.e. the sac-like mesocœle of van Wijhe, the walls of which give origin to the mesonephrotome or mesonephros. Further, according to van Wijhe, the dorsal part of the unsegmented metacœlom is itself segmented, but not, as in the case of the mesocœle, with respect to both splanchnopleuric and somatopleuric walls. The segmentation is manifest only on the somatopleuric side, and consists of a distinct series of hollow somatopleuric outgrowths, called by him hypomeres, which give rise to the pronephros and the segmental duct.

Van Wijhe considers that the whole metacœlom was originally segmented, because in the lower vertebrates the segmentation reaches further ventral-wards, so that in Selachia the body-cavity is almost truly segmental. Also in the gill-region of Amphioxus the cavities which are homologous with the body-cavity arise segmentally.

Fig. 156.—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.

As is well known, Balfour and Semper were led, from their embryological researches, to compare the nephric organs of vertebrates with those of annelids, and, indeed, the nature of the vertebrate segmental excretory organs has always been the fact which has kept alive the belief in the origin of vertebrates from a segmented annelid. These segmental organs thus compared were the mesonephric tubules, and doubts arose, especially in the mind of Gegenbaur, as to the validity of such a comparison, because the mesonephric tubules did not open to the exterior, but into a duct—the segmental duct—which was an unsegmented structure opening into the cloaca; also because the segmental duct, which was the excretory duct of the pronephros, was formed first, and the mesonephric tubules only opened into it after it was fully formed. Further, the pronephros was said to arise from an outbulging of the somatopleuric mesoblast, which extended over a limited number of metameres, and was not segmental, but continuous. Gegenbaur and others therefore argued that the original prevertebrate excretory organ was the pronephros and its duct, not the mesonephros, from which they concluded that the vertebrate must have been derived from an unsegmented type of animal, and not from the segmented annelid type.

Such a view, however, has no further reason for acceptance, as it was based on wrong premises, for Rückert has shown that the pronephros does arise as a series of segmental nephric tubules, and is not unsegmented. He also has pointed out that in Torpedo the anterior part of the pronephric duct shows indications of being segmented, a statement fully borne out by the researches of Maas on Myxine, who gives the clearest evidence that in this animal the anterior part of the pronephric duct is formed by the fusion of a series of separate ducts, each of which in all probability once opened out separately to the exterior.

Rückert therefore concludes that Balfour and Semper were right in deriving the segmental organs of vertebrates from those of annelids, but that the annelid organs are represented in the vertebrate, not by the mesonephric tubules, but by the pronephric tubules and their ducts, which originally opened separately to the exterior. By the fusion of such tubules the anterior part of the segmental duct was formed, while its posterior part either arose by a later cœnogenetic lengthening, or is the only remnant of a series of pronephric tubules which originally extended the whole length of the body, as suggested also by Maas and Boveri. Rückert therefore supposed that the mesonephric tubules were a secondary set of nephric organs, which were not necessarily directly derived from the annelid nephric organs.

At present, then, Rückert's view is the one most generally accepted—the original annelid nephric organs are represented by the pronephric tubules and the pronephric duct, not by the mesonephric tubules, which are a later formation. This latter statement would hold good if the mesonephric tubules were found entirely in segments posterior to those containing the pronephric tubules; such, however, is said not to be the case, for the two sets of organs are said to overlap in some cases; even when they exist in the same segments, the former are said always to be formed from a more dorsal part of the cœlom than the pronephros, always to be a later formation, and never to give any indication of communicating with the exterior except by way of the pronephric duct.