THE VASA EFFERENTIA. t 505 



this are four ureters (u]. In female embryos more than four 

 ureters are not usually to be seen in a single section. This is 

 probably owing to the persistence, in some instances, of the 

 intimate connection between the ureters found at an earlier 

 stage of development, and results in a single ureter coming 

 to serve as the collecting duct for several segments. A section 

 through a male embryo of stage P would mainly differ from 

 that through a female in the absence of the oviduct, and in the 

 presence of probably six 1 , instead of four, ureters. 



The exact amount of fusion which takes place between the 

 ureters, and the 'exact number of the ureters, cannot easily be 

 determined from sections, but the study of sections is chiefly 

 of value in shewing the general nature of the changes which take 

 place in the process of attaining the adult condition. 



It may be noticed, as a consequence of the above account, 

 that the formation of the ureters takes place by a growth of the 

 original segmental tubes, and not by a splitting off of parts of the 

 wall of the Wolffian duct. 



The formation of ureters in Scyllium, which has been only 

 very cursorily alluded to by Professor Semper, appears to differ 

 very considerably from that in Acanthias as narrated by him. 



The Vasa Efferentia. 



A comparison of the results of Professor Semper on Elasmo- 

 branchs, and Dr Spengel on Amphibians, suggests several 

 interesting questions with reference to the development of the 

 vasa efferentia, and the longitudinal canal of the Wolffian body. 



Professor Semper was the first to describe the adult anatomy 

 and development of vasa efferentia in Elasmobranchs, and 

 the following extracts will fully illustrate his views with reference 

 to them. 



" In 2 dem friihesten Stadium finden sich wie friiher angegeben 

 ungefahr 34 Trichter in der Leibeshohle, von diesen gehen die 

 27 hintersten in die persistirenden Segmentaltrichter iiber, von 

 denen 4 beim erwachsenen Thiere auf dem Mesorchium stehen. 



1 This at least holds good for one of my embryos at this stage, which is labelled 

 Scy. canicula, but which may possibly be Scy. stellare. 



2 Loc. cit. p. 364. 



B. 33 



