DEVELOPMENT OF THE HAIRS. 



329 



enlargement of their ends, which in their growth surround connective tissue 

 processes to form the hair-papilla. The embryonal connective tissue imme- 

 diately surrounding the epidermal ingrowth differentiates into the fibrous 

 sheath and the glassy membrane. 



Meanwhile and even before the formation of the papilla, the epithelial 

 contents of the young follicle differentiate into an axial strand of spindle 

 cells, that later undergoes keratinization and becomes the hair-shaft, which 

 A grows by subsequent additions from the ma- 



trix surrounding the papilla. In addition to 

 forming the outer root-sheath, the peripheral 

 elements contribute the matrix-cells that 

 occupy the fundus of the follicle and surround 

 the papilla. The cells covering the summit 

 and adjacent sides of the papilla are converted 

 into elongated spindles that gradually become 

 horny and assume the characteristics of the 

 cortical substance of the hair. When present, 







FIG. 373. Sections of developing 

 skin showing earliest stages in forma- 

 tion of hair-follicle; in D epithelial 

 cylinder is invading mesoderm. X 90. 



Hair-follicle 



Papilla 



FIG. 374. Developing skin, showing later stages of hair- 

 follicles ; mesoderm is forming hair-papilla and fibrous sheath 

 of follicle. X 90. 



the medulla is developed by the transformation of the cells occupying the sum- 

 mit of the papilla, which enlarge, become less granular and grow upwards as an 

 axial strand that invades the chief substance of the hair and accumulates 

 keratohyalin within its cells. The pigment particles, which appear later, 

 are first evident in the hair-bulb and probably arise within the epithelial tissue. 

 The elements of the hair-cuticle and of the inner root-sheath are differentiated 

 from the matrix-cells at the sides of the papilla. The tall columnar elements 

 become elongated and converted into the cornified plates of the cuticle both 

 of the hair and of the inner root-sheath. The layers of Huxley and of 

 Henle are derived from cells that soon exhibit granules of keratohyalin, so 

 that on reaching the level of the summit of the papilla the process of corni- 

 fication has been established. 



The growth of the hair takes place exclusively at the lower end of 

 its bulb, where, so long as the hair grows, the conversion of the matrix-cells 

 into the substance of the hair is continuously progressing. By this process 

 the substance already differentiated is pushed upwards by the cells undergoing 

 transformation and these, in turn, are displaced by the succeeding elements. 

 In this way, by the addition of new increments in its bulb, the hair is forced 

 onwards and, in the case of those first formed, through the epidermis that 

 still blocks the mouth of the follicle. This eruption begins on the scalp and 

 regions of the eyebrows about the fifth foetal month and on the extremities 

 about a month later. 



