612 DEVELOPMENT OF THE EGG IN HUMAN SPECIES. 



completely separated from the inner, by the disappearance of the 

 partition which for a time connected the two with each other (Fig. 

 209, c), this source of vascular supply is cut off; and the second 

 chorion cannot, therefore, remain vascular after that period. But 

 the third or permanent chorion, that is, the allantois, derives its ves- 

 sels directly from those of the foetus, and retains its connection with 

 them during the whole period of gestation. A chorion, therefore, 

 which is universally and permanently vascular, can be no other 

 than the allantois, converted into an external investing membrane 

 of the egg. 



Thirdly, the chorion, which is at one time, as we have seen, every- 

 where villous and shaggy, becomes afterward partially bald. This 

 change begins to take place about the end of the second month. 

 It commences at a point opposite the situation of the foetus and the 

 insertion of the foetal vessels. The villosities of this region cease 

 growing ; and as the entire egg continues to enlarge, the villosities 

 at the point indicated fail to keep pace with its growth, and with 

 the progressive expansion of the chorion. They accordingly be- 

 come at this part thinner and more scattered, leaving the surface 

 of the chorion comparatively smooth and bald. This baldness in- 

 creases in extent and becomes more and more complete, spreading 



and advancing over the adja- 

 cent portions of the chorion, 

 until at least two-thirds of its 

 surface have become nearly 

 or quite destitute of villosities. 

 At the opposite point of the 

 surface of the egg, however, 

 that portion, namely, which 

 corresponds with the insertion 

 of the foetal vessels, the villosi- 

 ties, instead of becoming atro- 

 phied, continue to grow ; and 

 this portion of the chorion be- 

 comes even more shaggy and 

 thickly set than before. The 



consequence is that the chorion afterward presents a very different 

 appearance at different portions of its surface. (Fig. 216.) The 

 greater part of it is smooth ; but a certain portion, constituting 

 about one-third of the whole, is covered with a soft and spongy 

 mass of long, thickly-set, compound villosities. It is this thickened 



HTMAN OVUM at end of third month : showing 

 placental portiou of thn choriou fully fo med. 



