FORMATION OF THE CIIORION. 613 



and shaggy 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 distinctly limited and separated from the 

 remainder by about the end of the third month. 



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

 respectively, with the atrophy and development of its villosities. 

 As the villosities shrivel and disappear over a part of its extent, 

 the looped capillary vessels, which they at first contained, disappear 

 also ; so that the smooth portion of the chorion shows afterward 

 only a few straggling vessels running over its surface, and does not 

 contain any abundant capillary plexus. In the thickened portion, 

 on the other hand, the vessels lengthen and ramify to an extent 

 corresponding with that of the villosities in which they are situated. 

 The allantoic arteries, coming from the abdomen of the foetus, enter 

 the villi, and penetrate through their whole extent ; forming, at the 

 placental portion of the chorion, a mass of tufted and ramified vas- 

 cular loops, while over the rest of the membrane they are merely 

 distributed as a few single and scattered vessels. 



The chorion, accordingly, is the external investing membrane of 

 the egg, produced by the consolidation and transformation of the 

 allantois. The placenta, furthermore, so far as it has now been 

 described, is evidently a part of the chorion ; that part, namely, 

 which is thickened, shaggy, and vascular, while the remainder is 

 comparatively thin, smooth, and membranous. 



