CHANGES IN THE OVUM. 



71 



» 



Chorion. 



The chorion^ the outer envelope proper to the ovum, is found covering 

 it loosely at the earliest period of its existence, and corresponds to the 

 membrane lining the ^gg in oviparous animals. It is a vast membraneous 

 sac, completely closed, and which, being moulded upon the uterine cavity, 

 resembles the uterus in form, having a body and two cornua ; the latter, 

 however, are not co extensive with those of the uterus. When the chorion 

 is distended, its cornua show fine and deep plicae or bulgings, like the 



Fig- 36. 



FtETAL Membranes of the Cow at Mid-term. 



A, Uterus opened on its left side ; B,B, Cotyledons of the Uterus ; C,C, Placentulse ; D,D, 

 Allantois ; E, Vesicle of the Urachus ; F, Amnion ; G, Umbilical Cord. 



caecum ; the cornua are always unequal in size, that in which the foetus 

 is developed being of course the largest. The external surface, otherwise 

 smooth, is studded with innumerable small, red, short papillae or " pro- 

 cesses," which are formed by the placental villous tufts. This papillary 

 face adheres slightly to the internal surface of the uterus, and between 

 the two surfaces a trifling quantity of brown or blood-colored fluid is 



