The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 157.—Transverse Section of Peripatus Embryo. (After von Kennel.)

Al., alimentary canal; N., nerve-cord; App., appendage; I, II, III, the three divisions (lateral, median, and dorso-median) of the cœlom.

Fig. 158.—Section of Peripatus. (After Sedgwick.)

Al., alimentary canal; N., nerve-cord; App., appendage; i.v., internal, and e.v., external vesicles of the segmented excretory tubule (coxal gland).

Here, then, in Peripatus, and presumably, therefore, in members of the Protostraca, we see that the original segmental organs of the annelid have become a series of nephric organs, which extended into the base of the appendages, and may therefore be called coxal glands; also it is clear, from Sedgwick's description, that if the appendages disappeared, the nephric organs would still remain, not as coxal glands, but as purely somatic excretory glands. They would still be homologous with the annelid segmental organs, or with the coxal glands, but would arise in toto from a part of the ventral cœlom or nephrocœle, more dorsal than the former appendicular part, because the appendages and their enclosed cœlom are always situated ventrally to the body. Again, according to Sedgwick, the nephric tubules are connected with two cœlomic vesicles, the one in the appendage the internal vesicle, and the other, the so-called bladder, or the external vesicle, in the body itself, close against the nerve-cord. Sedgwick appears to consider that either of these vesicles may form the end-sac of a nephric tubule, for he discusses the question whether the single vesicle, which in each case gives origin to the nephridia of the first three legs, corresponds to the internal or external vesicle. He decides, it is true, in favour of the internal vesicle, and therefore considers the excretory organ to be appendicular, i.e. a coxal gland, in these segments as well as in those more posterior. Still, the very discussion shows that in his opinion, at all events, the external vesicle might represent the end-sac of the tubule, in the absence of the internal or appendicular vesicle.

Such an arrangement as Sedgwick describes in Peripatus is the very condition required to give rise to the pronephric and mesonephric tubules, as deduced by me from the consideration of the vertebrate, and harmonizes and clears up the controversy about the mesonephros and pronephros in the most satisfactory manner. Both pronephros and mesonephros are seen to be derivatives of the original annelid segmental organs, not directly from an annelid, but by way of an arthropodan ancestor; the difference between the two is simply that the pronephric organs were coxal glands, and indicate, therefore, the presence of the original metasomatic appendages, while the mesonephric organs were homologous organs, formed in segments of later origin which had lost their appendages. For this reason the pronephros is said to be formed, in part at least, from a portion of the cœlom situated more ventrally than the purely somatic part which gives rise to the mesonephros. For this reason Sedgwick, Brauer, etc., can say that the mesonephros is strictly homodynamous with the pronephros; while equally Rückert, Semon, and van Wijhe can say it is not homodynamous, in so far that the two organs are not derived strictly from absolutely homologous parts of the cœlom. For this reason Semon can speak of the mesonephros as a dorsal derivative of the pronephros, just as Sedgwick says that the external or somatic vesicle of Peripatus is a derivative of the appendicular nephric organ. For this reason the pronephros, or rather a part of it, is always derived from the somatopleuric layer, for, as is clear from Miss Sheldon's drawing, the part of the cœlom in Peripatus which dips into the appendage is derived from the somatopleuric layer alone.

Such a cœlom as that of Peripatus, Fig. [157], would represent the origin of the vertebrate cœlom, and would therefore represent the procœlom of van Wijhe. In strict accordance with this, we see that it separates into a dorsal part, the walls of which give origin to the somatic muscles, or at all events to the great longitudinal dorsal muscles of the animal, and a ventral part, which forms a nephrocœle, dips into the appendage, and gives origin to the muscles of the appendage. In the vertebrate, after the somatic dorsal part or myocœle has separated off, a ventral part is left, which forms a nephrocœle in the trunk-region, and gives origin to the splanchnic striated muscles in the cranial region, i.e. to the muscles which, according to my theory, were once appendicular muscles. This ventral nephrocœlic part is divisible in the trunk into a segmented part, which forms the excretory organs proper, and an unsegmented part, the metacœle or true body-cavity of the vertebrate.

This comparison of the procœlom of the vertebrate and arthropod signifies that the vertebrate metacœle was directly derived by ventral downgrowth from the arthropod nephrocœle, so that if, as I suppose, the vertebrate nervous system represents the conjoined nervous system and alimentary canal of the arthropod, then the vertebrate metacœle, or body-cavity, must have been originally confined to the region on each side of the central nervous system, and from this position have spread ventrally, to enclose ultimately the new-formed vertebrate gut. This means that the body-cavity (metacœle) of the vertebrate is not the same as the body-cavity of the annelid, but corresponds to a ventral extension of the nephrocœle, or ventral part of such body-cavity.

Such a phylogenetic history is most probable, because it explains most naturally and simply the facts of the development of the vertebrate body-cavity; for the mesoblast always originates in the neighbourhood of the notochord and central nervous system, and the lumen of the body-cavity always appears first in that region, and then extends laterally and ventrally on each side until it reaches the most ventral surface of the embryo, thus forming a ventral mesentery, which ultimately disappears, and the body-cavity surrounds the gut, except for the dorsal mesentery. Thus Shipley, in his description of the formation of the mesoblastic plates which line the body-cavity in Ammocœtes, describes them as commencing in two bands of mesoblast situated on each side, close against the commencing nervous system:—