430 



MAMMALIA. 



and sub-zonal membrane unite, a false chorion is formed. Thus 

 where the placenta is formed is the true chorion, the other pait 

 the false chorion. 



From the outer surface of this chorion, which is highly vascu- 

 lar, villi grow out which fit into the crypts or pits in the uterus 

 {cr). These villi may become much branched. Not only are 

 the villi highly vascular, but the uterine pits are likewise. Here 



Fig. 204. — Vertical Section of injected Placenta of a Mare. 



ch, Chorion ; cr, uterioe crypts ; g, uteriue glands ; I', "blood-vessels ; E, epithelium. 

 (Turner.) 



there takes 2ilace that interchange of blood hy difi'usion between 

 the parent and foetus. 



The yolk or umbilical sac becomes smaller (fig. 205, uv), 

 and may atrophy before the close of uterine existence. The 

 allantoic stalk gives rise to two structures, the urinary bladder 

 and a cord, the uraclms, which connects the bladder with the 

 umbilicus. These membranes are shed at birth, and sometimes 



