734 GENERAL CONCLUSIONS. 



ducts may actually develop independently, as they do exception- 

 ally 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 Mullerian duct have been observed. 



In Amphibia (Salamandra) the permanent opening of the 

 Mullerian duct is formed independently, some way behind the 

 pronephros. 



In Elasmobranchii the original opening of the segmental 

 duct forms the permanent opening of the Mullerian 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 Mullerian 

 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. 



In Elasmobranchii, on the other hand, the functional opening 

 is formed at a period when we should expect the pronephros to 

 develop. This state is very possibly the result of a differenti- 

 ation by which the pronephros gradually ceased to become 

 developed, but one of its peritoneal openings remained as the 

 abdominal aperture of the Mullerian duct. Aves, finally, appear 

 to have become differentiated along a third line ; since in their 

 ancestors the anterior (?) pore of the head-kidney appears to 

 have become specialised as the permanent opening of the 

 Mullerian duct. 



The Mullerian duct is usually formed in a more or less com- 

 plete manner in both sexes. In Ganoids, where the separation 

 between it and the Wolffian duct is not completed to the cloaca, 

 and in the Dipnoi, it probably serves to carry off the generative 

 products of both sexes. In other cases however only the female 



