736 THE EVOLUTION OF THE PLACENTA. 



The polycotyledonary forms of placenta are due to similar 

 concentrations of the fcetal villi of an originally diffused pla- 

 centa. . 



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 pla- 

 centa is not, like that of the Carnivora, directly derived from a 

 type with both allantoic and umbilical vascularization of the 

 chorion. The discoidal and dome-shaped placentae of the 

 Armadillos, Myrmecophaga, and the Sloths may easily have been 

 formed from a diffused placenta, just as the discoidal placenta of 

 the Simiidae and Hominidae 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 Carnivora. A zonary placenta may be quite as easily de- 

 rived from a diffused placenta as from a discoidal placenta ; and 

 the presence of two villous patches at the poles of the chorion in 

 Elephas very probably indicates that its placenta has been evolved 

 from a diffused placenta. 



Although it would not be wise to attempt to found a classi- 

 fication upon the placental characters alone, it may be worth 

 while to make a few suggestions as to the affinities of the orders 

 of Mammalia indicated by the structure of the placenta. We 

 clearly, of course, have to start with forms which could not be 

 grouped with any of the existing orders, but which might be 

 called the Protoplacentalia. They probably had the primitive 

 type of placenta described above : the nearest living repre- 

 sentatives of the group are the Rodentia, Insectivora, and Chei- 

 roptera. Before, however, these three groups had become dis- 

 tinctly differentiated, there must have branched off from the 

 .primitive stock the ancestors of the Lemuridae, the Ungulata, 

 and the Edentata. 



It is obvious on general anatomical grounds that the Monkeys 

 and Man are to be derived from a primitive Lemurian type ; and 



