656 REPKODUCTION. 



tion, with its villosities, may be extricated from the maternal portion 

 without the laceration of either. 



In the carnivorous animals, a similar highly developed, vascular por- 

 tion of the allantois runs, in the form of a broad belt, round its middle ; 

 corresponding in situation with an equally developed zone of the uterine 

 mucous membrane. Here the foetal and maternal structures are mutu- 

 ally adherent ; while, elsewhere, over both extremities of the egg, they 

 lie simply in contact with each other. When gestation comes to an end, 

 and the foetus, with its membranes, is expelled, the thickened zone of 

 uterine mucous membrane is detached at the same time, its place being 

 afterward made good by a new growth. 



In man, as shown in the preceding chapter, the permanently thick- 

 ened portions of the chorion and decidua are united with each other, 

 by mutual interpenetration, in a flattened, cake-like mass of rounded 

 form, occupying 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 development of the placenta takes place in the following manner : 



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

 the uterine mucous membrane ; and are afterward developed into tufted 

 vascular ramifications, each of which turns upon itself in a loop at the 

 farther extremity of the villus. At the same time the uterine follicle, 

 into which the villus has penetrated, enlarges to a similar extent ; send- 

 ing out branching diverticula, corresponding with the 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 former enlarges. 



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

 complication of structure. The capillary blood-vessels between them 

 also become unusually developed by enlargement of their inoscula- 

 tions ; so that every follicle is covered with a network of dilated capil- 

 laries, derived from the blood-vessels of the original decidua. 



As the formation of the placenta goes on, the arrangement of the 

 foetal blood-vessels remains the same. They continue to form vascular 

 loops, penetrating deeply into the decidual membrane ; only they 

 become more elongated, and their ramifications more abundant and 

 tortuous. The maternal capillaries, however, on the outside of the 

 uterine follicles, are considerably altered in their anatomical relations. 

 They enlarge in all directions, and, encroaching upon the spaces between 

 them, fuse successively with each other ; losing in this way the form 

 of a capillary network, and becoming dilated into sinuses, which com- 

 municate freely with those in the walls of the uterus. As the original 

 capillary plexus occupied the entire thickness of the hypertrophied 

 decidua, the vascular sinuses, into which it is thus converted, are 

 equally extensive. They commence at the uterine surface of the 

 placenta, where it is in contact with the muscular walls of the organ, 



