CHORION. 749 



portion which is afterward concerned in the formation of the placenta; 

 while the remaining smooth portion continues to be known under the 

 name of the chorion. The placental portion of the chorion becomes dis- 

 tinctly limited in outline by about the end of the third month. 



The vascularity of the chorion keeps pace, in its different parts, with 

 the atrophy and development of its villosities. As the villosities shrivel 

 and disappear over a part of its extent, the bloodvessels with which they 

 were supplied diminish in abundance ; and the smooth portion of the 

 chorion finally shows only a few straggling vessels running over its sur- 

 face, but not connected with any abundant capillary plexus. In the 

 thickened portion, on the other hand, the bloodvessels lengthen and 

 ramify to an extent corresponding with that of the villosities in which 

 they are situated. The arteries, coming from the abdomen of the foetus, 

 divide into branches which enter the villi, and penetrate through their 

 whole extent ; forming, at the placental portion of the chorion, a mass 

 of tufted and ramified vascular loops, while the rest of the membrane 

 has a comparatively scanty vascular supply. 



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 foetal membranes, is a part of the 

 chorion, distinguished from the rest by the local development of its villi 

 and bloodvessels. 



