The Origin of Vertebrates

by Walter Holbrook Gaskell

"These two bands are separated dorsally by the juxtaposition of the dorsal wall of the mesenteron and the epiblast, and ventrally by the hypoblastic yolk-cells which are in contact with the epiblast over two-thirds of the embryo. Subsequently, but at a much later date, the mesoblast is completed ventrally by the downgrowth on each side of these mesoblastic plates. The subsequent downward growth is brought about by the cells proliferating along the free ventral edge of the mesoblast, these cells then growing ventralwards, pushing their way between the yoke-cells and epiblast."

The derivation of the vertebrate pronephric segmental organs from the metasomatic coxal glands of a primitive arthropod would mean, if the segmental organs of Peripatus be taken as the type, that such glands opened to the exterior on every segment, either at the base of the appendage or on the appendage itself. It is taken for granted by most observers that the pronephric segmental organs once opened to the exterior on each segment, and then, from some cause or other, ceased to do so, and the separate ducts, by a process of fusion, came to form a single segmental duct, which opened into the cloaca. Many observers have been led to the conclusion that the pronephric duct is epiblastic in origin, although from its position in the adult, it appears far removed from all epiblastic formations. However, at no time in the developmental history is there any clear evidence of actual fusion of any part of the pronephric organ with the epidermis, and the latest observer, Brauer, is strongly of opinion that there is never sufficiently close contact with the epidermis to warrant the statement that the epiblastic cells take part in the formation of the duct. All that can be said is, that the formation of the duct takes place at a time when the pronephric diverticulum is in close propinquity to the epidermis, before the ventral downgrowth of the myotome has taken place.

The formation of the anterior portion of the pronephric duct is, according to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly brought about by the fusion of a number of pronephric tubules, which, according to Maas, are clearly seen in the youngest specimens as separate segmental tubes; each of these tubules is supplied by a capillary network from a segmental branch of the aorta, as in the tubules of Amphioxus according to Boveri, and does not possess a glomerulus.

The posterior part of the duct into which the mesonephric tubules enter possesses also a capillary network, which Maas considers to represent the original capillary network of a series of pronephric tubules, the only remnant of which is the duct into which the mesonephric tubules open. He therefore argues that the pronephric duct indicates a series of pronephric tubules, which originally extended along the whole length of the body, and were supplanted by the mesonephric tubules, which also belonged to the same segments.

I also think that the paired appendages which have left the pronephric tubules as signs of their past existence, existed originally, in the invertebrate stage, on every segment of the body. But I do not consider that such a statement is at all equivalent to saying that such pairs of tubules must have existed upon every one of the segments existing at the present day; for it seems to me that Rückert is much more likely to be right when he says that in Selachians the duct clearly does grow back, and is not formed throughout in situ; so that he gives a double explanation of the formation of the duct—a palingenetic anterior part formed by the fusion of the extremities of the original excretory tubules, to which a posterior cœnogenetic lengthening has been added.

It does not seem to me at all necessary that the immediate invertebrate ancestor of the vertebrate should have possessed excretory organs which opened out separately to the exterior on each segment; a fusion may already have taken place in the invertebrate stage, and so a single duct have been acquired for a number of organs. Such a suggestion has been made by Rückert, because of the fact discovered by Cunningham and E. Meyer, that the segmental organs of Lanice conchilega are on each side connected together by a single strong longitudinal canal. I would, however, go further than this and say, that even although the nephric organs of the polychæte ancestor opened out on every segment, and although the primitive arthropodan ancestor derived from such polychæte possessed coxal glands which opened out either on to or at the base of each appendage, similarly to those of Peripatus, yet the immediate arthropodan ancestor, with its palæostracan affinities, may already have possessed metasomatic coxal glands, all of which opened into a single duct, with a single opening to the exterior.

Judging from Limulus, such was very probably the case, for Patten and Hazen have shown (1) that the coxal glands of Limulus are segmental organs belonging to the prosomatic segments; (2) that the organs belonging to the cheliceral and ectognathal segments are not developed; (3) that the four glands belonging to the endognaths become connected together by a stolon, which communicates with a single nephric duct, opening to the exterior on the basal segment of the 5th prosomatic appendage (the last endognath). At no time is there any evidence of any separate openings or any fusion with the ectoderm, such as might indicate separate openings of these prosomatic coxal segmental organs. Thus we see that in Limulus, which is presumably much nearer the annelid condition than the vertebrate, all evidence of separate nephric ducts opening to the exterior on each prosomatic segment has entirely disappeared, just as is the case in the metasomatic coxal glands (i.e. the pronephros) of the vertebrate. What is seen in the prosomatic region of Limulus, and doubtless also of the Eurypterids, may very probably have occurred in the metasomatic region of the immediate invertebrate ancestors of the vertebrate, and so account for the single pronephric duct belonging to a number of pronephric organs.

The interpretation of these various embryological investigations may be summed up as follows:—

1. The ancestor of the vertebrates possessed a pair of appendages on each segment; into the base of each of these appendages the segmental excretory organ sent a diverticulum, thus forming a coxal gland.

2. Such coxal glands, even in the invertebrate stage, may have discharged into a common duct which opened to the exterior most posteriorly.