EXCRETORY ORGANS. 693 



The next important change concerns the segmental duct, 

 which becomes longitudinally split into two complete ducts in 

 the female, and one complete duct and parts of a second duct in 

 the male. The manner in which this takes place is diagram- 

 matically represented in fig. 387 by the clear line x, and in 

 transverse section in figs. 388 and 389. The resulting ducts are 

 (i) the Wolffian duct or mesonephric duct (wd\ dorsally, which 

 remains continuous with the excretory tubules of the meso- 

 nephros, and ventrally (2) the oviduct or Miillerian duct in the 

 female, and the rudiments of this duct in the male. In the 



FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN 



ELASMOBRANCH EMBRYO. 



pd. segmental duct. It opens at o into the body cavity and at its other extremity 

 into the cloaca; x. line along which the division appears which separates the segmental 

 duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental 

 tubes. They open at one end into the body cavity, and at the other into the seg- 

 mental duct. 



female the formation of these ducts takes place (fig. 389) by a 

 nearly solid rod of cells being gradually split off from the 

 ventral side of all but the foremost part of the original segmental 

 duct. This nearly solid cord is the Miillerian duct (pd}. A 

 very small portion of the lumen of the original segmental duct 

 is perhaps continued into it, but in any case it very soon acquires 

 a wide lumen (fig. 389 A). The anterior part of the segmental 

 duct is not divided, but remains continuous with the Mullerian 

 duct, of which its anterior pore forms the permanent peritoneal 

 opening 1 (fig. 387). The remainder of the segmental duct (after 

 the loss of its anterior section, and the part split off from its 

 ventral side) forms the Wolffian duct. The process of formation 

 of these ducts in the male differs from that in the female chiefly 



1 Five or six segmental tubes belong to the region of the undivided anterior part 

 of the segmental duct, which forms the front end of the Mullerian duct ; but they ap- 

 pear to atrophy very early, without acquiring a definite attachment to the segmental 

 duct. 



