MAMMALIA. 431 



440. Integument. The skin, as in forms already de- 

 scribed, consists of two portions, an ectodermal portion, the 

 epidermis, and the dermis or true skin which is derived from 

 the mesoderm (Fig. 230). Hair is found in the young of all 

 mammals, though it may be wanting in the adult (as in 

 whales), or may occur only sparsely. Hair is produced by the 

 epidermis, but is nourished by a papilla of dermal tissue (Fig. 

 230, /) . Each hair consists of a central part, or pith, surrounded 

 by a denser cornified portion, the cortex. Hair differs much 

 in color and in structure^ from the soft fur of the seal to 

 the quills of the hedgehog and porcupine. To be considered 

 in the same connection with hair are the nails, claws, and hoofs, 

 the scales on the tail of the rat or beaver, and the horny ma- 

 terial of horns. 



441. Supplementary Studies for Field and Library. What is the 



economic value of the skins of mammals ? How are they prepared for the 

 uses to which they are put? What animals are prized for their hairy 

 products (fur, wool, etc.) ? What special qualities must the hair have 

 to be useful in making cloth? 



What instances can you adduce of advantageous coloring in the hair 

 of mammals? What variations of color may be found within a single 

 species? What changes of color are possible to a single individual? How 

 are these changes brought about? What peculiar qualities have the quills 

 of the porcupine? 



442. Integumentary Glands, derived from the epidermis, 

 are common in mammals. Associated with the hairs are the 

 oil glands. Over various parts of the body are long tubular 

 sweat glands buried in the dermis. The mammary glands, 

 which are characteristic of the group, are specially developed 

 skin glands, apparently more allied to the oil glands. They 

 are much lobed, and usually have teats or mammae ; but in the 

 monotremes these are wanting, and the young merely lick the 

 secretion from a " milk area." The glands may be distributed 

 along the entire abdominal surface (carnivora) or confined 

 either to the anterior (primates) or posterior portion (rumi- 

 nants). The number of the glands is correlated in a general 

 way with the number of young produced at a birth. 



