498 DEVELOPMENT OF ELASMOBRANCH FISHES. 



very obvious. In the first place, the ventral portion split off 

 from the segmental duct, instead of being as in the female 

 larger in front than the Wolffian duct, is very much smaller ; 

 while behind it does not form a continuous duct, but in some 

 parts a lumen is present, and in others again absent (PL 21, fig. 6). 

 It does not even form an unbroken cord, but is divided in dis- 

 connected portions. Those parts with a lumen do not appear to 

 open into the Wolffian duct. 



The process of splitting extends gradually backwards, so that 



there is a much longer rudimentary Mullerian duct by stage O 



than by stage N. By stage P the posterior portions of the 



Mullerian ducts have vanished. The anterior parts remain, 



as has been already stated, till adult life. A second difference 



between the male and female depends on the fact that, in the 



male, the splitting of the segmental duct into Mullerian duct 



and Wolffian duct never extends beyond the hinder extremity 



of the small intestine. A third and rather important point 



of difference consists in the splitting commencing far nearer 



the front end of the segmental duct in the male than in the 



female. In the female it was shewn that about 48 sections 



intervened between the front end of the segmental duct and 



the point where this became split, and that this region included 



five or six segmental tubes. In the male the homologous space 



only occupies about 7 to 12 sections, and does not contain the 



rudiment of more than a single segmental tube. Although my 



sections have not an absolutely uniform thickness, yet the above 



figures suffice to shew in a conclusive manner that the splitting 



of the segmental duct commences far further forwards in the 



male than in the female. This difference accounts for two facts 



which were mentioned in connection with the excretory organs 



of the adult, viz. (i) the greater length of the Wolffian body 



in the male than in the female, and (2) the fact that although a 



nearly similar number of segmental tubes persist in the adults 



of both sexes, yet that in the male there are five or six more 



segments in front of the first fully developed segmental opening 



than in the female. 



The above description of the formation of the Mullerian duct 

 in the male agrees very closely with that of Professor Semper 

 for Acanthias. For Scyllium however he denies, as it appears to 



