496 DEVELOPMENT OF ELASMOBRANCH FISHES. 



tubes in front of the Wolffian duct never become in the female 

 united with the segmental duct. The anterior end of the 

 Wolffian duct- is very much smaller than the oviduct adjoining 

 it, and as the reverse holds good in the male, an easy method is 

 afforded of distinguishing the two sexes even at the earliest 

 period of the formation of the Wolffian duct. 



Hitherto merely the general features of the development of 

 the oviduct and Wolffian duct have been alluded to, but a 

 careful inspection of any good series of sections, shewing the 

 junction of these two ducts, brings to light some features worth 

 noticing in the formation of the oviduct. It might have been 

 anticipated that, where the two ducts unite behind as the seg- 

 mental duct, their lumens would have nearly the same diameter, 

 but normally this appears to be far from the case. 



To illustrate the formation of the oviduct I have represented 

 a series of sections through a junction in an embryo in which 

 the splitting into two ducts had only just commenced (PI. 21, 

 fig. i), but I have found that the features of this series of 

 sections are exactly reproduced in other series in which the 

 splitting has extended as far back as the end of the small intes- 

 tine. In the series represented (PI. 21) I A is the foremost 

 section, and I D the hindermost. In I A the oviduct (o d} is as 

 large or slightly larger than the Wolffian duct (w. d}, and in the 

 section in front of this (which I have not represented) was con- 

 siderably the larger of the two ducts. In I B the oviduct has 

 become markedly smaller, but there is no indication of its lumen 

 becoming united with that of the Wolffian duct the two ducts, 

 though in contact, are distinctly separate. In I C the walls of 

 the two ducts have fused, and the oviduct appears merely as a 

 ridge on the under surface of the Wolffian duct, and its lumen, 

 though extremely minute, shews no sign of becoming one ivitJi 

 tJiat of the Wolffian dnct. Finally, in I D the oviduct can 

 merely be recognised as a thickening on the under side of the 

 segmental duct, as we must now call the single duct, but a slight 

 bulging downwards of the lumen of the segmental duct appears 

 to indicate that the lumens of the two ducts may perhaps have 

 actually united. But of this I could not be by any means 

 certain, and it seems quite possible that the lumen of the oviduct 

 never does open into that of the segmental duct. 



