C04 GENERAL CONCLUSIONS. 



At first the generative products seem to Lave been discharged 

 freely into the body cavity, and transported to the exterior by the 

 abdominal pores (vide p. 515). 



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 segmented 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 testicu- 

 lar products and secretion of the Wolfh'an 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 Miillerian duct and a dorsal 

 Wolffian duct. 



The Miillerian duct was probably continuous with one or more 

 of the abdominal openings of the pronephros which served as genera- 

 tive pores. At first the segmental duct was probably split longi- 

 tudinally into two equal portions, and this mode of splitting is 

 exceptionally retained in some Elasmobranchii ; but the generative 

 function of the Miillerian duct gradually impressed itself more and 

 more upon the embryonic development, so that, in the course of time, 

 the Miillerian 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 ottering 

 in this respect a less primitive condition than the Elasmobranchii ; 

 while in Aves it lias been carried even further, and it seems pos- 

 sible that in some Amniota the Miillerian and segmental ducts 

 may actua'ly develop independently, as they do exceptionally in 

 individual specimens of Salamandra (Fiirbringer). 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 Miillerian duct have been observed. 



In Amphibia (Salamandra) the permanent opening of the Miil- 

 lerian duct is formed independently, some way behind the pronephros. 



In Elasmobranchii the original opening of the segmental duct 

 forms the permanent opening of the Miillerian 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 Miillerian duct. 



