EXCRETORY ORGAN*. 571 



the horns of the cloaca, and also retires from its primitive position 

 between the epiblast and mesoblast, and assumes a position close to 

 the epithelium lining the body cavity (fig. 380, sd). The general fea- 

 tures of the excretory organs at this period are diagrammatically re- 

 presented in the woodcut (fig. 387). In this fig. pd is the segmental 

 duct and o its abdominal opening ; s.t points to the segmental tubes, 

 the finer details of whose structure are not represented in the diagram. 

 The mesonephros thus forms at this period an elongated gland com- 

 posed of a series of isolated coiled tubes, one extremity of each of 

 which opens into the body cavity, and the other into the segmental 

 duct, which forms the only duct of the system, and communicates at 

 its front end with the body cavity, and behind with the cloaca. 



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 diagrammatically represented in 

 fig. 387 bv the clear line x, and in transverse section in rigs. 388 and 389. 

 The resulting ducts are (I) the Wolffiau duct or mesonephric duct 

 (wd), dorsally, which remains continuous with the excretory tubules 

 of the mesonephros, and ventrally (2) the oviduct or Mtillerian 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 aud 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; g.t. segmeutal tubes. 

 They open at one end into the body cavity, and at the other into the segmental 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 (od~). 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 con- 

 tinuous with the Miillerian duct, of which its anterior pore forms the 

 permanent peritoneal opening 1 (fig. 387). The remainder of the seg- 



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 Miillerian duct: but they ap- 

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

 duct. 



