296 



HISTOGENESIS 



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 polygonal epidermal cells. About 

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

 mesenchyme produces the anlage of the hair papilla, which projects into 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 basal epidermal ceUs surrounding the hair 

 papilla. These cells give rise to a central core which grows toward the surface, 



Lpib uhium-~. 



s-T^c, 





Epidermal anlage of, ' ' " 







'^~^ Central cells 



' ^^ Epidermal anlage of 

 - ' - hair B 



\ - 



Epidermal anlage of hair C-ry 



v^^;^^' 1 }ila^€ of hair papilla 



Mesenchymal shralh — t^XpVS-,^,. 

 ______ • IwpiC 



Hair papilla' 



Fig. 300. — Section through the integument of the face of a 65 nun. human fetus showing three stages in 

 the early development of the hair. X 330. 



distinct from the peripheral cells which form the outer sheath of the hair (Fig. 301). 

 The central core of cells becomes the inner hair sheath and 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. 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 medulla. The 

 hair grows at the base and is pushed out through the central cavity of the anlage, 

 the cells of which degenerate. When the hair projects above the surface of the 

 epidermis it breaks and carries with it the epitrichial layer. The mesenchymal 



