The Origin of Vertebrates

by Walter Holbrook Gaskell

The recent observations of Brauer on the excretory organs of the Gymnophiona throw great doubt on the existence of mesonephric and pronephric tubules in the same segment. He criticizes the observations on which such statements are based, and concludes that, as in Hypogeophis, the nephrotome which is cut off after the separation of the sclero-myotome gives origin to the pronephros in the more anterior regions, just as it gives origin to the mesonephros in the more posterior regions. In fact, the observations of van Wijhe and others do not in reality show that two excretory organs may be formed in one segment, the one mesonephric from the remains of the mesomere and the other pronephric from the hypomere, but rather that in such cases there is only one organ—the pronephros—part of which is formed from the mesomere and part from the hypomere. Brauer goes further than this, and doubts the validity of any distinction between pronephros and mesonephros, on the ground of the former arising from a more ventral part of the procœlom than the latter; for, as he says, it is only possible to speak of one part of the somite as being more ventral than another part when both parts are in the same segment; so that if pronephric and mesonephric organs are never in the same segment, we cannot say with certainty that the former arises more ventrally than the latter.

These observations of Brauer strongly confirm Sedgwick's original statement that the pronephric and mesonephric organs are homodynamous organs, in that they are both derived from the original serially situated nephric organs, the differences between them being of a subordinate nature and not sufficient to force us to believe that the mesonephros is an organ of quite different origin to the pronephros. So, also, Price, from his investigations of the excretory organs of Bdellostoma, considers that in this animal both pronephros and mesonephros are derived from a common embryonic kidney, to which he gives the name holonephros.

Brauer also is among those who conclude that the vertebrate excretory organs were derived from those of annelids; he thinks that the original ancestor possessed a series of similar organs over the whole pronephric and mesonephric regions, and that the anterior pronephric organs, which alone form the segmental duct, became modified for a larval existence—that their peculiarities were adaptive rather than ancestral. This last view seems to me very far-fetched, without any sufficient basis for its acceptance. According to the much more probable and reasonable view, the pronephros represents the oldest and original excretory organs, while the mesonephros is a later formation. Brauer's evidence seems to me to signify that the pronephros, mesonephros, and metanephros are all serially homologous, and that the pronephros bears much the same relation to the mesonephros that the mesonephros does to the metanephros. The great distinction of the pronephros is that it, and it alone, forms the segmental duct.

We may sum up the conclusions at which we have now arrived as follows:—

1. The pronephric tubules and the pronephric duct are the oldest part of the excretory system, and are distinctly in evidence for a few segments only in the most anterior part of the trunk-region immediately following the branchial region. They differ also from the mesonephric tubules by not being so clearly segmental with the myotomes.

2. The mesonephric tubules belong to segments posterior to those of the pronephros, are strictly segmental with the myotomes, and open into the pronephric duct.

3. All observers are agreed that the two sets of excretory organs resemble each other in very many respects, as though they arose from the same series of primitive organs, and, according to Sedgwick and Brauer, no distinction of any importance does exist between the two sets of organs. Other observers, however, consider that the pronephric organs, in part at all events, arise from a part of the nephrocœle more ventral than that which gives origin to the mesonephric organs, and that this difference in position of origin, combined with the formation of the segmental duct, does constitute a true morphological distinction between the two sets of organs.

4. All the recent observers are in agreement that the vertebrate excretory organs strongly indicate a derivation from the segmental organs of annelids.

The very strongest support has been given to this last conclusion by the recent discoveries of Boveri and Goodrich upon the excretory organs of Amphioxus. According to Boveri, the nephric tubules of Amphioxus open into the dorsal cœlom by one or more funnels. Around each funnel are situated groups of peculiar cells, called by him 'Fadenzellen,' each of which sends a long process across the opening of the funnel. Goodrich has examined these 'Fadenzellen,' and found that they are typical pipe-cells, or solenocytes, such as he has described in the nephridial organs of various members of the annelid group Polychæta. Also, just as in the Polychæta, the ciliated nephric tubule has no internal funnel-shaped opening into the cœlom, but terminates in these groups of solenocytes. "Each solenocyte consists of a cell-body and nucleus situated at the distal free extremity of a delicate tube; the proximal end of the tube pierces the wall of the nephridial canal and opens into its lumen. A single long flagellum arising from the cells works in the tube and projects into the canal."

The exceedingly close resemblance between the organs of Amphioxus and those of Phyllodoce, as given in his paper, is most striking, and, as he says, leads to the conclusion that the excretory organs of Amphioxus are essentially identical with the nephridia of certain polychæte worms.