xm 



PHYLUM CHORDATA 



489 



the one sort very numerous and forming the soft fur, and those of 



the other consisting of longer and coarser hairs scattered over the 



surface. Examples 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 epi- 

 dermis. The first rudi- 

 ment of a developing hair 

 (Fig. 1102) usually takes 

 the form of a slight down- 

 wardly projecting out- 

 growth — the hair-germ 

 (grm.) — from the lower 

 mucous layer of the epi- 

 dermis, beneath which 

 there is soon discernible 

 a condensation of the 

 dermal tissue to form the 

 rudiment of a hair-papUla 

 (pp.). In some Mammals, 

 however, the dermal 

 papilla makes its appear- 

 ance before the hair-germ. 

 The hair-germ, which con- 

 sists 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-folliclo, becoming ar- 

 ranged in two layers, the 

 inner and outer root-sheaths 

 {sh\,sh\). The epidermal 

 cells in immediate con- 

 tact with the hair-papilla 

 retain their protoplasmic 

 character and form the 

 hair-hulh (bib.), by the 

 activity of which the further gxowth 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 

 VOL. II H H 



Fig. 1101. — Longitudinal section through a hair (dia- 

 grammatic). Ap, band of muscular fibres inserted 

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

 longitudinal, and JP". internal circular fibrous layer 

 of follicle ; Ft, fatty tissue in the dermis ; GH, hyaline 

 membrane between the root-sheath and the follicle ; 

 HBD, sebaceous gland ; HP. hair -papilla with vessels 

 in its intei-ior ; M. medullary substance (pith) of the 

 hair ; 0, cuticle ; R, cortical layer ; Sc, horny layer 

 of epidermis ; Srh. hair-shaft ; SM, Malpighian layer 

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

 root-sheath, (From Wiedersheim's Coruparative 

 Anatomy.) 



