904 



OF GENERATION ACTION OF THE FEMALE. 



chorion and its villi contain no vessels; and the fluid drawn in by the tufts 

 is communicated to the embryo by the absorbing powers of the germinal 

 membrane of the latter. But when the tufts are penetrated by bloodvessels, 

 and their communication with the embryo becomes more direct, the means 

 by which they communicate with the parent are found to be still essentially 

 the same ; namely, a double layer of nucleated cells, one layer belonging to 

 the foetal tuft, and the other to the vascular maternal surface. It is from 

 these elements that the Placenta is formed. 1 



754. The first stage in this process consists in the extension of the Foetal 

 vessels into the villi of the Chorion over its entire surface, in the manner 

 hereafter to be detailed ; so that the nutriment v/hich these villi imbibe, in- 

 stead of being merely added to the albuminous fluid surrounding the yolk- 

 bag, is now conveyed directly to the embryo. This the earliest and sim- 

 plest mode by which the Foetus effects a new connection with the parent 

 is the only one in which it ever takes place in the lower Mammalia, which 

 are hence properly designated as " uon-placental," rather than as ovo-vivip- 

 arous. In the higher Mammalia, however, there soon occurs a great exten- 

 sion of the vascular tufts of the foetal chorion, at certain points; and a cor- 

 responding adaptation, on the part of the uterine structure, to afford them 



FIG. 320. 



FIG. 321. 



Portion of the ultimate ramifications of thi um- 

 bilical vessels, forming the Foetal Villi of tlie Pla- 

 centa. 



an increased supply of nutritious fluid. 

 These specially prolonged portions are 

 scattered in the Rurniuautia and some 

 other Mammalia, over the whole sur- 

 face of the chorion, forming what are 

 termed the " cotyledons ;" but in the 

 higher orders, and in Man, they are 

 concentrated in one spot, forming the 

 Placenta. In some of the lower tribes 

 the maternal and the fcrtal portions 

 of the placenta may be very easily 

 separated ; the former consisting of 

 the thickened Decidua, and the latter 

 being composed of the prolonged and 

 ramifying muscular tufts of the Cho- 



Portion of one of the Foetal Villi, about to 

 form part <>f the Placenta, highly magnified: 

 a, a, its cellular covering ; l>, b, b, its looped ves- 

 sels ; c, c, its basis of connective tissue. 



I'lacenta varies in form to ;i considerable extent in different animals. In 

 Cetaeea and in many Ungulata, as the Horse and Zebra, the villi project from all 

 pirts of the. chorion, and their placenta is consequently termed d'lfuxf,. In Car- 

 nivora, in Seals, in the Elephant and Ilyrax, the villi are only developed in a ring or 

 zone around the chorion, and the placenta is therefore called "zonnry. In Ruminants 

 the villi are developed in certain spots, irregularly distributed over the chorion, form- 

 ing the ,-uhil,;l<nu>KN p lac entn. Finally, in Man and in Rodents the villi are limited 

 to a >ingb' spot, and the Placenta is thence termed diacnid. See Report of Turner's 

 Lectures delivered at the College of Surgeons, in Lancet for June, 1875. 



