152 THE URINOGENITAL ORGANS OF VERTEBRATES. 



mechanism for the transportation of the semen to the exterior 

 has not been completely made out. Analogy would certainly 

 lead us to expect the ureter to serve in Ganoids as the vas 

 deferens. 



The position of the generative ducts might in some cases 

 lead to the supposition that they are not Miillerian ducts, or, in 

 other words, the most anterior pair of segmental organs but 

 a pair of the posterior segmental tubes. 



What are the true homologies of the generative ducts of 

 Lepidosteus, which are continuous with the generative glands, 

 is somewhat doubtful. It is very probable that they may re- 

 present the similarly functioning ducts of other Ganoids, but 

 that they have undergone further changes as to their anterior 

 extremities. 



It is, on the other hand, possible that their generative ducts 

 are the same structures as those ducts of Osseous fishes, which 

 are continuous with the generative organs. These latter ducts 

 are perhaps related to the abdominal pores, and had best be 

 considered in connection with these; but a completely satisfac- 

 tory answer to the questions which arise in reference to them 

 can only be given by a study of their development. 



In the Cyclostomes the generative products pass out by an 

 abdominal pore, which communicates with the peritoneal cavity 

 by two short tubes 1 , and which also receives the ducts of the 

 kidneys. 



Gegenbaur suggests that these are to be looked upon as 

 Miillerian ducts, and as therefore developed from the segmental 

 ducts of the kidneys. Another possible view is that they are 

 the primitive external openings of a pair of segmental organs. 

 In Selachians there are usually stated to be a pair of abdominal 

 pores. In Scyllium I have only been able to find, on each side, 

 a large deep pocket opening to the exterior, but closed below 

 towards the peritoneal cavity, so that in it there seem to be no 

 abdominal pores 2 . In the Greenland Shark (Lcrniargits Borcalis) 



1 According to M tiller (Myxi>wit/cn, 1845) there is in Myxine an abdominal pore 

 with two short canals leading into it, and Vogt and Pappenheim (An. Sfi. A\tf. 

 Part iv. Vol. XI.) state that in Petromyzon there are two such pores, each connected 

 with a short canal. 



; My i>\\n rough examination of preserved specimens was hardly sufficient to 



