THE KIDNEYS. 319 



the tube into which they open is the commencement of the 

 Miillerian duct. 



Towards the end of the fifth day the two hinder pits close 

 up and disappear. The anterior pit persists, and forms the 

 peritoneal opening of the Miillerian duct or oviduct. The 

 Miillerian duct itself grows rapidly backwards ; it lies in close 

 contact with the outer wall of the Wolffian duct, and in its 

 hinder part appears to be formed from cells derived from the 

 wall of the Wolffian duct. 



About the end of the sixth day, the Miillerian duct has grown 

 backwards as far as the cloaca. It remains blind at its hinder 

 end in the male, but in the female opens, at a later stage, into 

 the cloaca. 



5. The Permanent Kidney, or Metanephros ; and the Ureter. 



The cloacal opening of the Wolffian duct (Fig. 123, TC) is 

 opposite the thirty-fourth somite, while the hinder end of the 

 Wolffian body itself does not extend behind the thirtieth somite. 

 The permanent kidney is formed in the mesoblast immediately 

 behind the Wolffian body, and is at first confined to the somites 

 from the thirty-first to the thirty-fourth inclusive, i.e. to the 

 somites between the hinder end of the Wolffian body and the 

 cloacal opening of the Wolffian duct. 



Towards the end of the fourth day, the ureter arises on each 

 side (c/. Fig. 123, KD) as a forwardly directed diverticulum from 

 the dorsal surface of the hinder end of the Wolffian duct. From 

 the ureter lateral outgrowths arise, which become continuous 

 with masses of cells in the mesoblast around it ; and from these 

 cells the tubules of the kidney, with their Malpighian bodies, 

 are formed at a later stage. The kidney is at first very small as 

 compared with the Wolffian body, but towards the close of incu- 

 bation it increases very considerably in size, growing forwards 

 dorsal to the Wolffian body. The ureters, which at first open 

 into the dorsal wall of the cloaca, through the hinder ends of the 

 Wolffian ducts (Fig. 123), acquire independent openings into 

 the cloaca about the sixth day. 



6. The Genital Ducts. 



a. In the male, or cock bird, the Miillerian ducts, although 

 they reach the cloaca, never open into it. In the later stages they 



