288 TOWARDS THE HUMAN FORM 



Mammals exhibit two stages, of which one is certainly primitive 

 and has led to the other. In the first the young develop 

 within the body of the mother in a special pocket, the womb, 

 formed at the expense of the oviducts and which simply served 

 to shelter them. They are born in an early stage of develop- 

 ment, and at once deposited in an external sub-ventral pocket 

 called the marsupium, containing mammae to which the } 7 oung 

 immediately attach themselves. These Mammals form the 

 sub-class Marsupialia, Didelphia, or Metatheria. They have 

 retained the epi-pubic bones of the Monotremes, but the 

 shoulder-girdle is singularly simplified. It is reduced to 

 clavicles and to scapulas, with which are united in the form of 

 an apophysis all that remains of the atrophied coracoids. 

 The two clavicles are never united into a furcula. The 

 posterior angle of the mandible is turned inward. 



The other Mammals form the sub-class Placentalia, Mono- 

 delphia, or Eutheria. The embryonic envelope of the young 

 and the maternal womb here enter into intimate union, 

 through the medium of highly vascular villi produced by 

 the embryo, which penetrate the uterine wall, to form, in 

 conjunction with it, the placenta, thus permitting the easy 

 filtration of the nutritive elements in the mother's blood into 

 the blood of the foetus. In the Eutherians the marsupial 

 bones have vanished, and the angle of the mandible is never 

 inflected. The placenta can be discoid (shaped like a cake), 

 zonary (shaped like a muff), bell-shaped, diffuse, or cotyledonary . 

 The same form of placenta characterizes an entire order. 



If, however, we attempt classification according to placenta 

 form, the Primates find themselves somewhat singularly 

 grouped with the Insectivora and the Rodent ; Elephants, 

 and the herbivorous Hyrax with the Carnivora, and Lemurs 

 with the Pachydermata, while the order of Edentata would 

 be broken up, for Orycteropus and the Armadillos have a zonary 

 placenta like Hyrax, the Ant-eater's is bell-shaped, and the 

 Pangolin's diffuse. The contact area of the allantois and 

 the chorion that furnishes the placental villi, is small in the 

 Insectivora and employed in its entirety in their formation. 

 It is extensive in the Primates where the villi are restricted 

 to a part of its surface only. There is here a considerable 

 difference, though it is not impossible that the one arrangement 

 may have developed from the other. In the other zoological 



