THE URINOGENITAL ORGANS OF VERTEKRATES. 139 



which divides the duct into two parts (fig. 3, wd. and md.). It 

 takes place in such a way that the front end of the segmental 

 duct, anterior to the entrance of the first segmental tube, together 

 with the ventral half of the rest of the duct, is split off from its 

 dorsal half as an independent duct (vide fig. 2, x). 



The dorsal portion also forms an independent duct, and into 

 it the segmental tubes continue to open. Such at least is the 



FIG. 3. TRANSVERSE SECTION OF A SELACHIAN EMBRYO ILLUSTRATING THE 

 FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGI- 

 TUDINAL SPLITTING OF THE SEGMENTAL DUCT. 



me. medullary canal ; mp. muscle-plate; ch. notochord; ao. aorta; cav. car- 

 dinal vein; st. segmental tube. On the one side the section passes through the 

 opening of a segmental tube into the body cavity. On the other this opening is 

 represented by dotted lines, and the opening of the segmental tube into the Wolfnan 

 duct has been cut through ; wd. Wolffian duct ; md, Miillerian duct. The Miil- 

 lerian duct and the Wolffian duct together constitute the primitive segmental duct ; 

 gr. The germinal ridge with the thickened germinal epithelium ; /. liver ; i. intes- 

 tine with spiral valve. 



for the female a few weeks before the publication of Semper's account but have not 

 yet made observations about the point for the male. 



My own previous account of the origin of the Wolffian duct (Quart. Journ. of 

 Micros. Science, Oct. 1874, and this edition, Paper V.), is completely false, and was 

 due to my not having had access to a complete series of my sections when I wrote the 

 paper. 



