XIII 



PHYLUM CHORDATA 



475 



of a hairy covering of this kind are seen in the case of the Platypus 

 and the Fur-Seals. 



A hair, like a feather, is formed from the epidermis. The first 

 rudiment of a developing 

 hair (Fig. 1113) usually takes 

 the form of a slight down- 

 wardly projecting out- 

 growth, the hair-germ (grm.), 

 from the lower mucous layer 

 of the epidermis, beneath 

 which there is soon dis- 

 cernible a condensation 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 fining of the hair- 

 follicle, becoming arranged 

 in two layers, the inner and W&- 1 -. 

 outer root- sheaths (sh.l, sh.2). 

 The epidermal cells in im- 

 mediate contact with the 

 hair-papilla retain their pro- 

 toplasmic character and 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 

 embedded in the follicle. 

 At the same time the follicle 



FIG. 1112. Longitudinal section through a hair (dia- 

 grammatic). Ap, band of muscular fibres inserted 

 into the hair-follicle ; Co. dermis ; F. external 

 longitudinal, and F'. internal circular fibrous layer 

 of follicle ; Ft, fatty tissue in the dermis ; GH, hya- 

 line membrane between the root-sheath and the 

 follicle ; HBD, sebaceous gland ; HP. hair-papilla 

 with vessels in its interior ; M. medullary substance 

 (pith) of the hair ; O, cuticle ; R, cortical layer ; 

 Sc, horny layer of epidermis ; SM , Malpighian layer 

 of epidermis ; WS, WS', outer and inner layers of 

 root-sheath. (From Wiedersheim's Comparative 

 Anatomy.) 



grows downwards into the 

 dermis. During its growth the hair is nourished by the blood- 

 vessels in the dermal hair-papilla, which projects into its base. 



G G 2 



