MEMBRANES OF THE IMPREGNATED EGG. 649 



chorion finally shows only a few straggling vessels running over its 

 surface, without any abundant capillary plexus. In the thickened por- 

 tion, on the other hand, the blood-vessels lengthen and ramify to an 

 extent corresponding with that of the villosities in which they are 

 situated. The arteries, coming from the foetus, divide into branches 

 which penetrate the villi throughout ; forming, at the placental por- 

 tion of the chorion, a mass of tufted and ramified vascular loops, while 

 in the rest of the membrane the vascular supply is comparatively 

 scanty. 



The chorion, accordingly, is the external investing membrane of the 

 egg, produced by an outgrowth from the body of the embryo ; and 

 the placenta, so far as it consists of the fcetal membranes, is a part 

 of the chorion, distinguished by the local development of its villi and 

 blood-vessels. 



