FORMATION OF THE CHORTON. 



611 



Fig. 215. 



Extremity of VILLOSITT op 

 CHORION, more highlj magni- 

 fied ; showing the arrangement of 

 bloodvessels in its interior. 



by vascular absorption, instead of the slow process of imbibition, 

 which has heretofore taken place through the comparatively incom- 

 plete and structureless villi of the cho- 

 rion. The capillary vessels, accordingly, 

 with which the chorion is supplied, begin 

 to penetrate into the substance of its vil- 

 losities. They enter the base or stem of 

 each villosity, and, following every divi- 

 sion of its compound ramifications, finally 

 reach its rounded extremities. Here they 

 turn upon themselves in loops (Fig. 215), 

 like the vessels in the papilla of the skin, 

 and retrace their course, to unite finally 

 with the venous trunks of the chorion. 



The villi of the chorion are, therefore, 

 very analogous in structure to those of 

 the intestine ; and their power of absorp- 

 tion, as in other similar instances, corre- 

 sponds with the abundance of their ramifications, and the extent 

 of their vascularity. 



It must be remembered, also, that these vessels all come from the 

 abdomen of the foetus ; and that whatever substances are taken up 

 by them are transported directly to the interior of the embryo, and 

 used for the nourishment of its tissues. The chorion, therefore, as 

 soon as its villi and bloodvessels are completely developed, becomes 

 an exceedingly active organ in the nutrition of the foetus ; and con- 

 stitutes, in fact, the only means by which new material can be in- 

 troduced from without. 



The existence of this general vascularity of the chorion affords 

 also, as Coste was the first to point out, a striking indication that 

 this membrane is in reality identical with the allantois of the 

 lower animals. If the reader will turn back to the illustrations of 

 the formation of the amnion and allantois (Chap. IX.), he will see 

 that the first chorion or investing membrane is formed exclusively 

 by the vitelline membrane, which is never vascular and cannot be- 

 come so by itself, since it has no direct connection with the fcetus. 

 The second chorion is formed by the union of the vitelline mem- 

 brane with the outer lamina of the amniotic fold. Both lamina 

 of the amniotic fold are at first vascular, since they are portions of 

 the external blastodermic layer, and derive their vessels from the 

 integument of the fcetus. But after the outer lamina has become 



