306 



HISTOGENESIS 



The hair is differentiated from the basal epidermal cells surrounding the hair 

 papilla. These cells elongate and grow centrally toward the surface distinct 



Epitrichium 



Hair bulb 

 Hair papilli 



-<2-g *&-E_^J^- 

 ~-^s3S?~^ 



FIG. 2g$. Section through the integument of the face of a 65 mm. embryo showing three stages in the 



early development of the hair. X 330. 



from the peripheral cells which form the outer sheath of the hair (Fig. 294). The 

 central core of cells gives rise to the inner hair sheath and to the shaft of the hair. 



At the sides of the outer hair 

 sheath two swellings appear on 

 the lower side of the obliquely 

 directed hair anlage. The more 

 superficial of these is the anlage 

 of the sebaceous gland (Fig. 294). 

 The deeper swelling is the "epi- 

 thelial bed, " a region where the 

 cells by rapid division con- 

 tribute to the growth of the 

 hair follicle. 



Superficial to the bulb, the 

 cells of the hair shaft become 

 cornified and differentiated into 

 outer cuticle, middle cortex and 

 central medulla. The inner 

 hair sheath extends from the 



Inner hair 

 Sheath 



Outer ha! i 

 Sheath 



Arrector pili 



ft 'fff^ muscle fiber* 



'Sebaceous gland. 



Mesenchymal 

 sheath 



Epithelial bed 



Root of hair 



Hair bulb 

 papilla 



FIG. 2g^. Longitudinal section through a developing hair 

 from a five and one-half months' fetus (Stohr). 



