72 HISTORY OF THE HUMAN BODY 



known as the chorion frondosum in distinction from the smooth 

 area, the chorion Iceve. A diffuse placenta, where the villi 

 cover the entire external surface of the chorion, is the most 

 primitive type, and is found in pigs, horses, whales and por- 

 poises; if a small portion of the chorion is left smooth, the 

 placenta is bell-shaped, as in some edentates and lemurs. By 

 a continuation of this process, that is, by a farther extension 

 of the smooth area, the placenta becomes discoidal, which is 

 the form characteristic of Man and the higher anthropoids, 

 insectivores, bats and rodents ; in the lower monkeys there are 

 two such discs, placed at opposite poles, the placenta discoidea 

 duplex. It is the single discoidal type, as found in man, that 

 gave the name " placenta " to this organ, as the word signi- 

 fies a round, flat cake. 



If there are two smooth areas at opposite poles, with pla- 

 cental villi between them, the zonary placenta is formed, the 

 type characteristic of all carnivores, elephants, Hyrax and the 

 Sirenia. A very distinct type of placentation is the cotyle- 

 donal, characteristic of ruminants. Here the placental struc- 

 ture is confined to small nodules or cotyledons scattered over 

 the entire surface of the chorion, and varying in number from 

 three to five in the deer to more than a hundred in the sheep 

 and cow. 



All of the above forms of placentation are easily derived 

 from the primitive diffuse type, and as a rule actually pass 

 through the changes during early development, the form finally 

 assumed being attained through the growth of smooth areas 

 (chorion lave). 



The methods of placentation may be again divided with 

 reference to the relationship to the uterine mucous membrane ; 

 in one type the villi at birth are simply drawn out of the ma- 

 ternal portion, leaving pits, and in the other type the union 

 between fetal and maternal elements is more intimate and the 

 separation occurs between the mucous and muscular walls of 

 the uterus itself, thus involving the loss of maternal mucous 

 membrane, called in this connection, the decidua. The latter 

 of these types, in which the placenta becomes a far more spe- 



