THE CHORION. 663 



(4) The Chorion is the external covering of the ovum. At first 

 it consists simply of the zona pellucida or vitelline membrane, 

 and then it is called the primitive chorion. Later, however, it is 

 supplemented by the part of the somatopleure removed from the 

 embryo in the process of forming the amnion. This blends with 

 the primitive chorion and strengthens it, and while lying beneath 

 the zona pellucida, receives the name of the subzonal membrane. 



FIG. 256. 



Diagram of an embryo, showing the relationship of the vascular allantois to the villi 

 of the chorion. (Cadiat.) a, Lies in cavity of the amnion under the embryo. 6, Yelk 

 sack, c, Allantois. rf, Vessels of the allantois dipping into the villi of the chorion. e, 

 Chorion. 



The chorion at first is a smooth membrane, but villous processes 

 early grow out from it. These villi are chiefly developed at its 

 upper part, where they aid in the formation of the foetal pla- 

 centa. 



The allantois, when it has spread over the chorion, becomes 

 blended with this membrane, and fills the villous processes with 

 the blood vessels it contains. 



