THE EXCRETORY SYSTEM. 421 



vitelline veins may persist, but more usually the right one becomes 

 much reduced in size, or else atrophies completely. 



In the rabbit, the vitelline circulation is of much less impor- 

 tance than in the chick, inasmuch as the nutrition of the rabbit 

 embryo is effected, not by the yolk-sac, but by the placenta. 



DEVELOPMENT OF THE EXCRETORY SYSTEM. 



The general history of the development of the excretory 

 organs and their ducts in the rabbit is much like that of the 

 chick. 



Xo trace of a head kidney, or pronephros, has yet been de- 

 scribed, and it may be assumed that this structure is either 

 altogether absent, or else very small and rudimentary. A seg- 

 mental, or Wolffian duct is early formed along each side of the 

 body ; and in connection with each duct a Wolffian body is 

 developed, which is large in the embryo, but which becomes 

 replaced functionally by the metanephros or permanent kidney 

 in the later stages and in the adult animal. The Miillerian 

 duct develops rather later than the Wolffian duct and Wolffian 

 body ; it lies very close to the Wolffian duct, but is apparently 

 independent of this. 



1. The Wolffian Duct. 



The mode of development of the Wolffian duct in the rabbit 

 has been much debated ; the point in dispute being whether it 

 is formed from mesoblast, or directly from the external epiblast. 



According to the observations of Hensen, supported by 

 Flemming, the Wolffian duct arises, early in the ninth day, as a 

 solid ridge-like thickening of the epiblast (Fig. 164, kc), at the 

 level of the fourth and fifth mesoblastic somites, and close to their 

 outer borders. It soon separates from the epiblast, and then lies 

 as a solid rod of cells between the epiblast and mesoblast ; this 

 rod grows rapidly backwards, becomes tubular by the formation 

 of an axial cavity or lumen, and on the eleventh day reaches 

 the hinder end of the body, and opens into the dorsal surface of 

 the allantois, just in front of the union of the rectum and the 

 allantois to form the cloaca (cf. Fig. 150, KC, tc). 



There is no doubt that the Wolffian duct, in the early stages 

 of its development, lies very close indeed to the epiblast, espe- 



