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 Zchzdua, the cilia of the eyelids and the 
tactile vibrissve 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 (AZanzs); 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, 
