MAMMALIA. 261 



extent in complexity. From the diffused placenta covering the 

 whole surface of the chorion, differentiations appear to have 

 taken place in various directions. The metadiscoidal placenta of 

 Man and Apes, from its mode of ontogeny (p. 248), 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 

 concentrations 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 fcetal 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 umbilical vascularization of the 

 chorion. The discoidal and dome-shaped placentae of the 

 Armadilloes, Myrmecophaga, and the Sloths may easily have 

 been formed from a diffused placenta, just as the discoidal 

 placenta of the Simiadae and Anthropidae appears to have been 

 formed from a diffused placenta like that of the Lemuridae. 



The presence of zonary placentas in Hyrax and Elephas does 

 not necessarily afford any proof of affinity of these types with 

 the Carnivora. 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 

 placenta is capable of being used to some extent in classification, 

 yet at the same time the striking resemblances which can exist 

 between such essentially different forms of placenta, as for 

 instance those of Man and the Rodentia, are likely to prevent it 

 being employed, except in conjunction with other characters. 



