MAMMALIA. 217 



yolk-sack still plays a part, though physiologically an unimportant 

 part, in rendering the chorion vascular. 



In the Carnivora again we have to start from the discoidal pla- 

 centa, as shewn by the fact that the allantoic region of the placenta 

 is at first discoidal (p. 20G). A zonary deciduate placenta indicates 

 an increase both in area and in complexity. The relative diminution 

 of the breadth of the placental zone in late foetal life in the zonary 

 placenta of the Carnivora is probably due to its being on the whole 

 advantageous to secure the nutrition of the foetus by insuring a more 

 intimate relation between the foetal and maternal parts, than by 

 increasing their area of contact. The reason of this is not obvious, 

 but as mentioned below, there are other cases where it can be shewn 

 that a diminution in the area of the placenta has taken place, ac- 

 companied by an increase in the complexity of its villi. 



The second type of differentiation from the primitive form of 

 discoidal placenta is illustrated by the Lemuridse, the Suida?, and 

 Manis. In all these cases the area of the placental villi appears to have 

 increased so as to cover nearly the whole subzonal membrane, without 

 the villi increasing to any great extent in complexity. From the 

 diffused placenta covering the whole surface of the chorion, differ- 

 entiations appear to have taken place in various directions. The 

 metadiscoidal placenta of Man and Apes, from its mode of ontogeny 

 (p. 203), is clearly derived from a diffused placenta — very probably 

 similar to that of Lemurs — by a concentration of the foetal villi, which 

 are originally spread over the whole chorion, to a disc-shaped area, 

 and by an increase in their arborescence. 



The polycotyledonary forms of placenta are due to similar con- 

 centrations of the foetal villi of an originally diffused placenta. 



In the Edentata we have a group with very varying types of 

 placenta. Very probably these may all be differentiations within 

 the group itself from a diffused placenta, such as that found in Manis. 

 The zonary placenta of Orycteropus is capable of being easily derived 

 from that of Manis, by the disappearance of the foetal villi at the 

 two poles of the ovum. The small size of the umbilical vesicle in 

 Orycteropus indicates that its discoidal placenta is not, like that in 

 Carnivora, directly derived from a type with both allantoic and um- 

 bilical vascularization of the chorion. The discoidal and dome-shaped 

 placentiB of the Armadilloes, Myrmecophaga, and the Sloths may 

 easily have been formed from a diffused placenta, just as the discoidal 

 placenta of the Simiadse and Anthropidse appears to have been 

 formed from a diffused placenta like that of the Lemuridae. 



The presence of zonary placentae in Hyrax and Elephas does not 

 necessarily afford any proof of affinity of these types with the Car- 

 nivora. A zonary placenta may quite easily be derived from a 

 diffused placenta; and the presence of two villous patches at the 

 poles of the chorion in Elephas indicates that this was very probably 

 the case with the placenta of this form. 



Although it is clear from the above considerations that the 



