Genital ducts. So far the origin and development of the excretory organs have been considered without reference to the modifications introduced by the excretory passages coming to serve as generative ducts. Such an unmodified state of the excretory organs is perhaps found permanently in Cyclostomata[265] and transitorily in the embryos of most forms.
At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (vide p. [626]).
The secondary relations of the excretory ducts to the generative organs seem to have been introduced by an opening connected with the pronephridian extremity of the segmental duct having acquired the function of admitting the generative products into it, and of carrying them outwards; so that primitively the segmental duct must have served as efferent duct both for the generative products and the pronephric secretion (just as the Wolffian duct still does for the testicular products and secretion of the Wolffian body in Elasmobranchii and Amphibia).
The opening by which the generative products entered the segmental duct can hardly have been specially developed for this purpose, but must almost certainly have been one of the peritoneal openings of the pronephros. As a consequence (by a process of natural selection) of the segmental duct having both a generative and a urinary function, a further differentiation took place, by which that duct became split into two—a ventral Müllerian duct and a dorsal Wolffian duct.
The Müllerian duct was probably continuous with one or more of the abdominal openings of the pronephros which served as generative pores. At first the segmental duct was probably split longitudinally into two equal portions, and this mode of splitting is exceptionally retained in some Elasmobranchii; but the generative function of the Müllerian duct gradually impressed itself more and more upon the embryonic development, so that, in the course of time, the Müllerian duct developed less and less at the expense of the Wolffian duct. This process appears partly to have taken place in Elasmobranchii, and still more in Amphibia, the Amphibia offering in this respect a less primitive condition than the Elasmobranchii; while in Aves it has been carried even further, and it seems possible that in some Amniota the Müllerian and segmental ducts may actually develop independently, as they do exceptionally in individual specimens of Salamandra (Fürbringer). The abdominal opening no doubt also became specialised. At first it is quite possible that more than one pronephric abdominal funnel may have served for the entrance of the generative products; this function being, no doubt, eventually restricted to one of them.
Three different types of development of the abdominal opening of the Müllerian duct have been observed.
In Amphibia (Salamandra) the permanent opening of the Müllerian duct is formed independently, some way behind the pronephros.
In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Müllerian duct, and no true pronephros appears to be formed.
In Birds the anterior of the three openings of the rudimentary pronephros remains as the permanent opening of the Müllerian duct.
These three modes of development very probably represent specialisations of the primitive state along three different lines. In Amphibia the specialisation of the opening appears to have gone so far that it no longer has any relation to the pronephros. It was probably originally one of the posterior openings of this gland.