COMPARATIVE ANATOMY OF MAMMALS. 659 



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 derived 

 from the outer or ectodermic layer of the embryo, 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 vibrissa; of the lips and cheeks. 



It is generally believed that the hairs of Mammals are homologous 

 with the feathers of Birds and the scales of Reptiles, but Maurer main- 

 tains that the facts of development upset the homology and point rather 

 to a resemblance between hairs and the sensory papillae of Amphibians. 

 But this is still under discussion. 



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. A few Mammals, 

 such as the Arctic fox, the mountain hare, and the ermine, become white 

 in winter, harmonising with the snow. In the case of Ross's lemming, 

 the cold is the stimulus evoking this change, which depends in great 

 part on the appearance of gas bubbles inside the hairs. 



That the colouring is sometimes of protective advantage we have 

 already noticed ; but in many cases no utilitarian interpretation can be 

 read into the stripes and markings. Those of related species often 

 form regular series, and are superficial outcrops of constitutional changes 

 hardly to be analysed. Sometimes there is considerable change during 



