454 



MAMMALIA. 



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

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

 the false chorion. 



From the outer surfain! nf this chorion, which is liighly vascu- 

 lar, ^■illi grow out which fit into the crypts or pits in the uterus 

 (cr). These villi may become much branched. Xot only are 

 the ■i'illi highly vascular, but the uterine pits are likewise. Here 



Fin. 236. — VioKTlCAL .'Skction ok iNJKrxKD Placent.^ of a 5Iare. 



€h, Cliorittii ; cr, uterine cypts ; fjy uterine glands ; V, blood-vt-ssels ; E, epithelium. 

 (Turner.) 



there takes place that interchange of blood by diftu.'iion between 

 the parent and foetus. 



The yolk or umbilical sac becomes smaller (fig. '237, ur), 

 and may atrophy before the close of uterine existence. The 

 allantoic stalk gives rise to two structures, the urinary bladder 

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

 umbilicus. These membranes are shed at birth, and sometimes 



