DEVELOPMENT OF THE HAIE 211 



The Hair Germ. The hairs arise at any time after the third 

 month of fetal life, their earliest anlage appearing as a slightly 

 increased proliferation of the cells of the germinal layer of the 

 epidermis. The further multiplication of the cylindrical cells 

 produces a solid columnar ingrowth of the epidermis which pene- 

 trates into, and sometimes through, the primitive derma. The 

 spheroidal cells of the intermediate layer of the epidermis increase 

 in size, assume a vesicular character, and finally by fatty degenera- 

 tion form the epidermal hair canal through which the future hair 

 reaches the surface. 



The Hair Column. The columnar epidermal ingrowths, hair 

 columns or hair pegs, come into early relation with the anlage of 

 the hair papilla which is formed by a proliferation of the mesen- 

 chymal cells at the tip of the hair column. Further development 

 of the papilla produces an indentation of the advancing hair 

 column and gives rise to a true dermal papilla of considerable 

 size. 



The Hair Bulb. Coincident with the formation of the papilla 

 there is an increased proliferation of the cells of the hair column 

 by which it is surrounded, and which therefore represents the 

 future hair bulb. Two other swellings appear in the hair column ; 

 one, the more superficial, forming the anlage of the sebaceous 

 gland, and the other, the deeper, forming the so-called matrix of 

 the hair which stands in close relation with the future regenera- 

 tion of the hair. 



The development of the hair papilla produces a slight evagi- 

 nation of the epithelium of the hair bulb, which is just sufficient 

 to redirect the growth of central cells of the hair column toward 

 the cutaneous surface. It is thus that the younger cells which 

 arise by mitosis in the germinal layers of the hair bulb are pushed 

 outward along the axis of the hair column where they form the 

 shaft of the future hair. The growth of the hair from the germi- 

 nal cells of the hair bulb is accompanied by beginning cornification 

 of the newly formed cells of the primitive hair shaft and of the 

 intermediate cells of the hair column. The growth of the shaft 

 is, however, preceded by enlargement, vesiculation, and fatty de- 

 generation of the central cells of the hair column, thus producing 

 a central canal through which the hair may grow, and which later 

 becomes continuous with the hair canal of the epidermis. 



The Hair Follicle. At this stage the hair column has been 

 differentiated into a peripheral follicle, the primitive root sheath, 



