THE PLACENTA. 



757 



its membranes, is expelled, the thickened zone of uterine mucous mem- 

 brane is detached at the same time, and its place is afterward made 

 good by a new growth. 



In the human species, as shown in the preceding chapter, the perma- 

 nently thickened portions of the chorion and decidua, united with each 

 other by mutual interpenetration and growth, form a single, flattened, 

 cake-like mass of rounded form, oceupj'ing rather less than one-third 

 of the surface of the chorion, and corresponding to a similar extent of 

 the inner surface of the uterus. This mass, consisting of the foetal and 

 maternal elements combined, is the placenta. 



The complete development of the placenta takes place in the follow- 

 ing manner : 



The villi of the chorion, when first formed, penetrate into follicles 

 situated in the substance of the uterine mucous membrane; and after 

 becoming vascular, they are developed into tufted ramifications of 

 bloodvessels, each one of which turns upon itself in a loop at the ex- 

 tremity. At the same time the uterine follicle, into which the villus 

 has penetrated, enlarges to a similar extent ; sending out branching 

 diverticula, corresponding with the multiplied ramifications of the 

 villus. The growth of the follicle and that of the villus thus go on 

 simultaneously and keep pace with each other ; the latter constantly 

 advancing as the cavity of the former enlarges. 



But it is not only the uterine follicles which increase in size and in 

 complication of structure at this period. The capillary bloodvessels, 

 which lie between them and ramify over their exterior, also become 

 unusually developed. They enlarge and inosculate freely with each 

 other ; so that every uterine folli- 



cle is covered with a network of 

 dilated capillaries, derived from 

 the bloodvessels of the original 

 decidua. 



As the formation of the pla- 

 centa goes on, the anatomical ar- 

 rangement of the foetal bloodves- 

 sels remains the same. They 

 continue to form vascular loops, 

 penetrating deeply into the de- 

 cidual membrane ; only they be- 

 come more elongated, and their 

 ramifications more abundant and 

 tortuous. The maternal capilla- 

 ries, however, on the outside of 



Fig. 272. 



Extremity of a FOETAL TUFT, from the 

 human placenta at term, in its recent condi- 

 tion. a, a. Capillary bloodvessels. Magnified 

 135 diameters. 



the uterine follicles, become con- 

 siderably altered in their anatomi- 

 cal relations. They enlarge in 



all directions, and, by encroaching upon the spaces situated between 

 them, fuse successively with each other ; and, losing gradually in this 



