THE HAIR 



295 



The first evidence of a hair anlage is the elongation of a cluster of 

 epidermal cells in the inner germinal layer (Fig. 300 ^4). The bases of 

 these cells project into the corium, and, above them, cells of the epidermis 

 are arranged parallel to the surface. The elongated cells continue to grow 

 downward until a cylindrical hair anlage is produced (Fig. 300 B, C). 

 This consists of an outer wall, formed of a single layer of columnar cells, 

 continuous with the basal layer of the epidermis. This wall bounds a 

 central mass of irregularly polyhedral epidermal cells. About the hair an- 

 lage the mesenchyma forms a sheath, and at its base a condensation of 

 mesenchyme produced the anlage of the hair papilla, which projects into 



Epitrichium 



Epidermal anlage of, 

 hair A 



Anlage of hair papilla 



^-Central cells 



Epidermal anlage of 

 hair B 



Anlage of hair papilla 



Epidermal anlage of hair C 



Mesenchymal sheath 



Hair bulb 



Hair papilla 



FIG. 300. ' Section through the integument of the face of a 65 mm. human fetus, showing three 

 stages in the early development of the hair. X 330. 



the enlarged base of the hair anlage. As development proceeds, the 

 hair anlage grows deeper into the corium and its base enlarges to form the 

 hair bulb (Fig. 300 C). The hair differentiates from the based epidermal 

 cells surrounding the hair papilla. These cells give rise to a central core 

 which grows toward the surface, distinct from the peripheral cells which 

 form the outer sheath of the hair (Fig. 301). The central core of cells be- 

 comes the inner hair sheath and the shaft of the hair. Two swellings of the 

 outer hair sheath appear on the lower side of the obliquely directed hair 

 anlage. The more superficial of these is the anlage of the sebaceous gland 

 (Fig. 301). The deeper swelling is the epithelial bed, a region where the 

 cells by rapid division contribute. to the growth of the hair follicle. 



Superficial to the bulb, the cells of the hair shaft become cornified 

 and differentiated into an outer cuticle, middle cortex, and central incon- 

 stant medulla. The hair grows at the base and is pushed out through the 



