Memoir: The Study of Nephridia and Genital Ducts Since 1895

Abstract

From the analysis given above of recent work on the development of the nephridia in the Oligochaeta, it may safely be concluded: that it is now definitely established that the whole nephridium from funnel to pore (including the whole nephridiostome, the excretory canal, the bladder and duct to the exterior, but excepting an insignificant ingrowth of epidermis at the pore in higher forms), arises from a special cell, the nephridioblast ; that the nephridioblast does not belong to the coelomic epithelium, is not derived from the coelomesoblastic band, but is of retroperitoneal origin, primitively occupies an intersegmental position, and only breaks through the coelomic epithelium at the point where the nephridiostome comes to open (Text-fig. 79). Further, it is exceedingly probable, though not so definitely proved, that such nephridioblasts are derived in the higher Oligochaeta (Megadrili) from nephric bands or rows of cells formed by the repeated forward division of posterior nephric teloblasts situated at first in the surface layer of ectoblast; or, if not from regular bands, at all events from originally superficial cells of ectodermal or ectomesodermal nature. Usually, and possibly always, the nephridial canal develops as a string of cells, produced by the repeated backward division of the nephridioblast, which string becomes hollowed out by an intracellular lumen. The nephridiostome is formed by the division of the residual nephridioblast itself, though some of the most anterior cells of the string may contribute to it. In this connexion it must be remembered that, like the larval protonephridia of the Polychaeta, the larval nephridia of the first segment in Oligochaeta appear certainly not to be developed from the coelomesoblast, but from ectoblastic or ectomesoblastic cells (cells 2c in Bimastus, according to Svetlov (1928)). They appear very early and in Tubifex even before the coelomesodermal teloblast has begun to divide (Ivanow, 1928). It may be concluded that, as long ago maintained by Vejdovsky, neither the embryonic nor the later nephridia are derived from the coelomesoblast.