The coexistence of abdominal pores and generative ducts in Mormyrus appears to me to demonstrate that the generative ducts in Teleostei cannot be derived from the coalescence of the investment of the generative organs with the abdominal pores.
Ganoidei. The true excretory gland of the adult Ganoidei resembles on the whole that of Teleostei, consisting of an elongated band on each side—the mesonephros—an anterior dilatation of which probably represents the pronephros.
There is in both sexes a Müllerian duct, provided, except in Lepidosteus, with an abdominal funnel, which is however situated relatively very far back in the abdominal cavity. The Müllerian ducts appear to serve as generative canals in both sexes. In Lepidosteus they are continuous with the investment of the generative glands, and thus a relation between the generative ducts and glands, very similar to that in Teleostei, is brought about.
Fig. 396. Section through the trunk of a Lepidosteus embryo on the sixth day after impregnation.
mc. medullary cord; ms. mesoblast; sg. segmental duct; ch. notochord; x. subnotochordal rod; hy. hypoblast.
Posteriorly the Müllerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each side is clearly the primitive segmental duct. It receives the secretion of a certain number of the posterior mesonephric tubules, and usually unites with its fellow to form a kind of bladder, opening by a single pore into the cloaca, behind the anus. The duct which receives the secretion of the anterior mesonephric tubules is the true mesonephric or Wolffian duct.
The development of the excretory system, which has been partially worked out in Acipenser and Lepidosteus[254], is on the whole very similar to that in the Teleostei. The first portion of the system to be formed is the segmental duct. In Lepidosteus this duct is formed as a groove-like invagination of the somatic peritoneal epithelium, precisely as in Teleostei, and shortly afterwards forms a duct lying between the mesoblast and the epiblast ([fig. 396], sg). In Acipenser (Salensky) however it is formed as a solid ridge of the somatic mesoblast, as in Petromyzon and Elasmobranchii ([fig. 397], Wg).
