720 MAMMALIA. 



of the vestibule lies the clitoris, a small rod-like body the 

 homologue of the penis. On the dorsal wall lie two small 

 Cowper's glands, and there are also perineal glands as in 

 the male. 



The fertilised egg develops within the uterus, and in the 

 rabbit, as in all Eutherian Mammals, the allantois of the 

 embryo becomes intimately connected with the wall of the 

 uterus to form the vascular placenta, the organ by means of 

 which the nutrition and respiration of the embryo are pro- 

 vided for. In the rabbit, and in other Rodents, there is, 

 before the development of the allantoic placenta, a pro- 

 visional yolk-sac placenta a structure of similar function 

 but of much less morphological complexity. The details 

 of the placentation of Mammals will be considered later. 



NOTES ON COMPARATIVE ANATOMY OF MAMMALS 



Skin. This consists of a superficial epidermis (ecto- 

 dermic) and of a subjacent mesodermic dermis or cutis. 



The most characteristic modification of the mammalian 

 epidermis is the hair. Each hair arises from the cornifica- 

 tion of an ingrowing epidermic papilla of the Malpighian 

 stratum of the epidermis, surrounded at its base by a moat- 

 like follicle, and nourished during growth by a vascular 

 projection of the dermis. 



Each hair consists of a spongy central part and a denser cortex, but 

 there are many diversities of form and structure, such as short fur and 

 long tresses, the soft wool of sheep and the bristles of pigs, the spines of 

 hedgehog, porcupine, and Echidna, the cilia of the eyelids and the 

 tactile vibrissae of the lips and cheeks. 



The hair keeps the animal dry and warm ; in the practically hairless 

 Cetacea the layer of fat or blubber underneath the skin also serves to 

 sustain the temperature of the body. Like feathers, hairs die away 

 and are cast off, being replaced by fresh growths. 



Among other tegumentary structures are the scales which 

 occur along with hairs on the pangolins (Mam's); the 

 scales on the tails of rats and beavers and some other forms ; 

 the thickened skin-pads or callosities on the ischia of apes, 

 the breast of camels, the legs of horses ; the nails, claws, or 

 hoofs which ensheath the ends of the digits in all Mammals 

 except Cetaceans. Unique is the armature of the armadillos. 



