482 



MAMMALIA. 



may mention the terrestrial habit and the absence of gills, 

 the presence of an amnion and allantois, the universal pre- 

 sence and relative importance of the hyomandibular cleft, the 

 presence of a primitive streak, of twelve pairs of cranial nerves, 

 the absorption of the persistent part of the mesenephros into the 

 testis, the presence of a ureter, the abortion in the adult 

 female of the mesonephric duct, and lastly the complete dis- 

 appearance of the conus arteriosus and the breaking up of the 

 ventral aorta. 



By these important characters the reptiles, birds, and mammals 



are sharply marked off 

 from both Amphibia 

 and Pisces ; the more 

 we consider them, the 

 more difficult it be- 

 comes to understand 

 on what grounds the 

 contention has been 

 made that mammals 

 are more closely related 

 to Amphibia than to 

 reptiles and have been 

 directly derived from 

 them in evolution. 



We shall now pro- 

 ceed to give some 

 account of such fea- 

 tures of mammalian 

 anatomy as seem to re- 

 quire a fuller explana- 

 tion to enable the reader to comprehend the detailed descriptions 

 of the orders which follow. Space does not permit of our giving 

 anything like a complete account of mammalian morphology. 

 For such we must refer the reader to special works on Anatomy 

 and Embryology. 



Hairs are to mammals what feathers are to birds. They are 

 never entirely absent ; even Cetacea and the hippopotamus 

 which seem to be without them, possess hairs on certain parts 

 even if only a few short bristles on the lips. Hairs are cornified 

 modifications of the epidermis. The bulbous root is placed 



FIG. 257. Section of human skin (from "Wiedersheim. 

 Co dermis (corium, cutis) ; D sebaceous glands ; 

 F subcutaneous fat ; G bloodvessels of dermis ; 

 OP vascular papilla of hair ; H hair ; N nerves in 

 dermis; #Pf nerve ending in dermis ; Se horny layer 

 of epidermis (stratum corneum), SD sweat gland ; 

 SD' duct of sweat gland ; SM malpighian layer of 

 epidermis (stratum 'malpighii). 



