648 REPRODUCTION. 



reached such an activity of growth that it requires to be supplied with 

 nourishment by means of vascular absorption, instead of the slow pro- 

 cess of imbibition hitherto taking place through the villi of the chorion. 

 The capillary blood-vessels, with which the chorion is supplied, pene- 

 trate its villosities. They enter the base of each villus, and following 

 the division of its compound ramifications, reach the extremities of its 

 terminal offshoots. Here they turn upon themselves in loops (Fig. 

 201), and retrace their course, to unite finally with the venous trunks 

 of the chorion. 



The villi of the chorion are, accordingly, analogous in structure and 

 function to those of the intestine ; their power of absorption corre- 

 sponding with the abundance of their ramifications, and the extent 

 of their vascularity. 



The blood-vessels of the chorion, furthermore, are derived from the 

 fetus ; and all substances absorbed by them are transported to the 

 interior of the body, and used for the nourishment of its tissues. 

 The chorion, therefore, as soon as its villi and blood-vessels are com- 

 pletely developed, becomes an essential organ for the nutrition of the 

 fetus, and the only means by which new material is introduced from 

 without. 



The third change of importance in the chorion is that after being at 

 first shaggy throughout, it becomes partially bald. (Fig. 199.) This 

 change begins about the end of the second month, commencing at the 

 point farthest from the insertion of the fetal blood-vessels. The vil- 

 losities of this region cease growing ; and while the entire chorion con- 

 tinues to enlarge, they fail to keep pace with its progressive expansion. 

 They accordingly become at this part thinner and more scattered, 

 leaving the surface comparatively bald. This baldness increases in 

 extent, spreading over the adjacent portions of the chorion, until at 

 least two-thirds of its surface have become nearly or quite clear of 

 villosities. 



At the opposite pole of the egg, namely, that which corresponds with 

 the insertion of the fetal blood-vessels, the villosities of the chorion, 

 instead of becoming atrophied, continue to grow ; and this portion 

 becomes even more shaggy and thickly set than before. The conse- 

 quence is that the chorion afterward presents a different appearance in 

 different regions. The greater part is smooth ; bat a certain portion, 

 constituting about one-third of the whole, is covered with long, thickly- 

 set, compound villosities. It is this thickened portion which is con- 

 cerned in the formation of the placenta; while the remainder continues 

 to be known under the name of the chorion. The placental portion of 

 the chorion becomes distinctly limited in outline by about the end 

 of the third month. 



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

 the development of its villosities. As the villosities shrivel and dis- 

 appear over a part of its extent, the blood-vessels with which they 

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



