462 



ZOOLOGY 



SECT, 



seb 



a conden&ttbn of the dermal tissue to form the rudiment of a 

 hair papilla (pp.). In some Mammals, however, the dermal papilla 

 makes its appearance before the hair germ. The hair germ, which 

 consists of a solid mass of epidermal cells, elongates, and soon 

 its axial portion becomes condensed and cornified to form the 

 shaft of the hair, while the more peripheral cells go to form the 

 lining of the hair-follicle, becoming arranged in two layers, the 

 inner and outer root sheaths (sA 1 ., sA, 2 .). The epidermal cells in 

 immediate contact with the hair papilla retain their proto- 

 plasmic character 



A B an d form the hair- 



bulb (bib), by the 

 activity of which 

 the further growth 

 of the hair is 

 effected. Soon the 

 upper end of the 

 hair shaft grows out ' 

 beyond the surface 

 of the epidermis, and 

 the projecting part 

 eventually becomes 

 much longer than 

 that which lies im- 

 bedded in the fol- 

 licle. At the same 

 the follicle 

 downwards 

 dermis. 



During its growth 

 the hair is nourished 

 by the blood-vessels 

 in the dermal hair- 

 papilla, which pro- 

 jects into its base. 



Modifications of 

 the hairs are often 

 found in certain 



parts. Such modified hairs are the elongated hairs of the tails 

 of some Mammals, e.g., most Ungulates ; the eye-lashes of the 

 eye-lids, which are stronger than the ordinary hairs ; and sensitive 

 hairs or vibrissce about the snout. In some Mammals the hairs 

 in part assume the form of spines, viz., in Echidna, the Hedgehogs, 

 and the Porcupines. 



The coating of hairs is sca/ity in some Mammals, and is 

 virtually absent in the Cetacea and Sirenia. In such cases the 

 skin is greatly thickened, as in the Elephants, &c., or, as in the 



time 

 grows 

 into the 



FIG. 1039. Four diagrams of stages in the development of a 

 hair. A , earliest stage in one of those Mammals in which 

 the dermal papilla appears first ; B, C, J), three stages in 

 the development of the hair in the human embryo, bib. 

 hair-bulb ; cm, horny layer of the epidermis ; foil, hair- 

 follicle ; grm. hair-germ ; h. extremity of hair projecting on 

 the surface ; muc. Malpighiaii layer of epidermis ; pp. dermal 

 papilla ; seb. developing sebaceous glands ; sh l . s/i 2 . inner 

 and outer root-sheaths. (After Hertwig.) 



