Development of the Fourth Day 235 



both Mullerian ducts degenerate and become 

 almost completely obliterated. 



The metanephros or permanent kidney. — The 

 permanent kidneys begin to develop towards 

 the end of the fourth day, in the mesoblast 

 between the hinder end of the Wolffian body 

 and the cloaca. The posterior end of the 

 Wolffian body is in the thirtieth somite, while 

 the cloacal opening of the Wolffian duct is 

 opposite the thirty-fourth somite, so that 

 there is a space of three or four somites be- 

 tween the two points ; it is in this space that 

 the first trace of the permanent kidney appears. 



Like the mesonephros, the first part of the 

 metanephros to be formed is its duct, the 

 ureter. The ureter is formed as an anteriorly 

 directed diverticulum from the dorsal side of 

 the posterior end of the Wolffian duct. It 

 grows forwards on the outer side of the mass 

 of mesoblast that lies behind the Wolffian 

 body, and, for a time, opens, as is evident 

 from its method of formation, into the hinder 

 end of the Wolffian duct : but on the sixth 

 day it acquires a separate opening into the 

 cloaca. From the ureter lateral outgrowths 

 arise, and these, becoming connected with 

 rods of tissue in the surrounding mesoblast, 



