EXCRETORY ORGANS. 605 



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 pro- 

 bably 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 de- 

 velop. This state is very possibly the result of a differentiation by 

 which the pronephros gradually ceased to become developed, but 

 one of its peritoneal openings remained as the abdominal aperture of 

 the Miillerian duct. Aves, finally, appear to have become differen- 

 tiated 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 Miillerian duct. 



The Miillerian duct is usually formed in a more or less complete 

 manner in both sexes. In. Ganoids, where the separation between it 

 and the Wolffiau 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 products pass out by it, and 

 the partial or complete formation of the Miillerian duct in the male 

 in these cases needs to be explained. This may be done either by 

 supposing the Ganoid arrangement to have been the primitive one 

 in the ancestors of the other forms, or, by supposing characters 

 acquired primitively by the female to have become inherited by both 

 sexes. 



It is a question whether the nature of the generative ducts of 

 Teleostei can be explained by comparison with those of Ganoids. 

 The fact that the Miillerian ducts of the Teleostean Ganoid Lepi- 

 dosteus attach themselves to the generative organs, and thus acquire 

 a resemblance to the generative ducts of Teleostei, affords a power- 

 ful argument in favour of the view that the generative ducts of 

 both sexes in the Teleostei are modified Miillerian ducts. Embry- 

 ology can however alone definitely settle this question. 



In the Elasmobranchii, Amphibia, and Amniota the male pro- 

 ducts are carried off by the Wolffian duct, and they are transported 

 to this duct, not by open peritoneal funnels of the mesonephros, but 

 by a network of ducts which sprout either from a certain number of 

 the Malpighian bodies opposite the testis (Amphibia, Amniota), or 

 from the stalks connecting the Malpighian bodies with the open 

 funnels (Elasmobranchii). After traversing this network the semen 

 passes (except in certain Anura) through a variable number of the 

 segmental tubes directly to the Wolfrian duct. The extent of the 

 connection of the testis with the Wolffian body is subject to great 

 variations, but it is usually more or less in the anterior region. 

 Rudiments of the testicular network have in many cases become 

 inherited by the female. 



The origin of the connection between the testis and Wolffian body is still 

 very obscure. It would be easy to understand how the testicular products, 



