REPTILES AND MAMMALS. 143 



discoidal cleavage resulting in a Disc-gastrula ; the eggs of 

 the latter are very small, and their unequal cleavage results 

 in the formation of a Hood-gastrula. Finally, two charac- 

 ters entirely peculiar to Mammals, and by which these 

 are distinguished both from Birds and Reptiles and from all 

 other animals, are the presence of a complete diaphragm, 

 and of the milk-glands (mamniai), by means of which tho 

 new-born young are nourished by the milk of the mother. 

 It is only in Mammals that the diaphragm forms a transverse 

 partition-wall across the body-cavity (coeloma), completely 

 separating the chest from the ventral cavity. (Cf. Plate V. 

 Fig. 16 2.) It is only among Mammals that the mother 

 nourishes the young with her milk ; and the whole class are 

 well named from this. 



These important facts in Comparative Anatomy and 

 Ontogeny clearly show that the tribe of Amnion Animals 

 (Amniota) bifurcated from the very first into two main 

 diverging lines ; on the one side, the Eeptilian line, from 

 which the Birds afterwards developed; on the other side, 

 the Mammalian line. The same facts also prove as indu- 

 bitably that Man originated from the latter line. For Man, 

 in common with Mammals, shares all the characteristics we 

 have mentioned, and is distinguished by them from all 

 other animals. And, finally, these facts indicate as certainly 

 those advances in vertebrate structure by which one branch 

 of the Primitive Amnion Animals developed into the parent- 

 form of Mammals. The most prominent of these advances 

 were (1) the characteristic modification of the skull and 

 brain ; (2) the formation of a covering of hair ; (3) the com- 

 plete development of the diaphragm ; and (4) the formation 

 of the milk-glands and the adaptation to the suckling of 



