518 EMBRYOLOGY OF THE CHICK 



The most posterior five or six fuse a little later and form the pygostyle 

 (Fig. 418). 



THE EXCRETORY SYSTEM 



The anterior tubules of the Wolffian body disappear before the end 

 of the fourth day, while the posterior tubules have increased in size and 

 become convoluted. The intermediate cell mass from which they arise 

 is quite prominent. In cross-sections the convoluted tubules will nat- 

 urally be cut at all angles, but they can be distinguished from the duct 

 by observing their much thicker walls. The glomeruli can also be seen 

 filled with blood vessels. 



The permanent kidney, or metanephros (Fig. 307), begins its de- 



Fig. 307. 



Diagram of Urogenital Organs. A, in Indifferent stage. B, development of 

 the male from the indifferent anlagen, and C, development in the female from the 

 indifferent anlagen. 



The dotted lines represent the organs in their relative positions in the adult 

 stage with the exception of the Miillerian duct in the male and the mesonephric 

 duct in the female. These latter ducts disappear for the most part. (After 

 Hertwig. ) 



velopment toward the fourth day in the region lying between the 

 Wolffian body and the cloaca, that is, between the thirtieth and thirty- 

 fourth segment. 



The metanephric duct or ureter forms first, as did the ducts of the 

 pronephros and the mesonephros. This duct grows forward on the outer 

 side of the mesoderm lying in the region just mentioned. It grows 

 from the dorsal side of the posterior end of the Wolffian duct anteriorly. 

 Naturally it has an opening into the Wolffian duct from which it is a 

 diverticulum, but on the sixth day it develops a separate opening into 

 the cloaca. 



It is from these ureters that lateral outgrowths arise which join 

 with the rods of tissue now forming in the surrounding mesoderm. 

 These outgrowths then develop into the tubules and Malpighian bodies 

 of the metanephros in a similar manner to the way the Wolffian bodies 

 developed. 



The permanent kidney is quite small when compared with tlie 

 metanephros but increases in size to a considerable extent just before 

 hatching. 



