EXCRETORY ORGANS. 733 



excretory organs is perhaps found permanently in Cyclosto- 

 mata 1 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 pronepJiric secretion (just 

 as the Wolffian duct still does for the testicular products and 

 secretion of the Wolffian body in Elasmobranchii and Am- 

 phibia). 



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 

 Mullerian duct and a dorsal Wolffian duct. 



The Mullerian 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 Mullerian duct gradually 

 impressed itself more and more upon the embryonic develop- 

 ment, so that, in the course of time, the Mullerian duct 

 developed less and less at the expense of the Wolffian duct. 

 This process appears partly to have taken place in Elasmo- 

 branchii, 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 Mullerian and segmental 



1 It is by no means certain that the transportation outwards of the genital products 

 by the abdominal pores in the Cyclostomata may not be the result of degeneration. 



