DEVELOPMENT OF HAIR IN THE F(ETUS. 



425 



going On outside, the cells within the follicle which at first were uniform in appearance, 

 are found to have undergone changes, having become transformed into the layers 

 of the root-sheath and the miniature conical hair. The inner root-sheath, lying next 

 to the hair (fig. 486, k), is distinguished by its translucency from the more opaque 

 outer part that fills up the rest of the cavity. The hair and the inner root-sheath 

 are formed from the Qells which cover the top of the papilla. These appear to 

 acquire eleidin-granules, and also, in the case of those which produce the hair, pig- 



Fig. 486. DEVELOPING HAIR FROM A HUMAN EMBRYO OF 4J MONTHS. (Ranvier.) 



p, papilla ; /, hair-rudiment ; i, cells forming inner root-sheath ; Jc, keratinized portion of inner root- 

 sheath, remaining uncoloured by carmine ; e, external root-sheath ; b, epithelial bed, for insertion of 

 arrector pili ; s, sebaceous gland ; t, sebaceous matter forming independently in the part which will 

 become the neck of the follicle. 



Fig. 487. REPLACEMENT OF OLD HAIR BY A NEWLY DEVELOPING ONE IN THE HUMAN SCALP. 



(Ranvier. ) 



p, papilla of the new hair ; i, its inner root-sheath ; e, its outer root-sheath continuous above with 

 that of the old hair, p' ; r, epithelial projection at the level of the insertion of the arrector pili, in. 



ment-granules ; the eleidin-granules are stated to be most abundant in the cells 

 which form the medulla of the hair and in those which form the inner root-sheath. 



As the young hair reaches in its growth the upper part of the follicle, the central 

 cells which block the neck of the follicle undergo a kind of fatty degeneration, and 

 disintegrate to produce a sebaceous secretion like that of the sebaceous glands (fig. 

 486, t). The latter in the meantime are becoming formed as extensions of the 

 outer root-sheath of the hair laterally into the corium (s), and they soon open into 

 the neck of the follicle. 



Between the soft cells of the hair-bulb in the growing hair there are frequently 

 seen wander-cells, which contain pigment-granules, and it is supposed that the pig- 



