THE SKIN AND ITS APPENDAGES. 



279 



root, but at the anterior border of the lunula ; from this point for- 

 ward the nail remains of constant thickness, since it derives no aug- 

 mentation in its passage over the nail-bed. The regeneration of 



FIG. 313. 



''^v??^'-^- 



- >x -i.siiiii^j^'^ 



Section of skin of foetal kitten, showing ecto- 

 dermic tissue (a) starting to grow into mesoderm 

 (t>) as solid epithelial process. 



Section of skin of foetal kitten, showing earliest 

 stage of development of hair : a, epidermis ex- 

 hibiting thickening and elevation of surface ; 6, 

 mesodermic tissue, showing indications of con- 

 densation. 



the nail after disease or injury depends upon the integrity of the 

 deeper layers of the epithelium. 



The development of the hair in the foetus proceeds entirely 

 from the ectoderm. The first indication of the process, about the 

 end of the third month, ap- 

 pears as a localized prolif- 

 eration of the ectodermic 

 cells, resulting in a slight 

 transient elevation of the sur- 

 face, and, at the same time, 

 in a feeble encroachment on 

 the subjacent mesoderm. 

 This ectodermal projection 

 soon becomes an epithelial 

 cylinder, whose expanded 

 club-shaped extremity pene- 

 trates deeply into the primi- 

 tive corium to form the epi- 

 thelial constituents of the 

 future hair-follicle. The dif- 

 ferentiation of the surrounding 

 connective tissue produces the 

 fibrous root-sheath, while 

 a projection opposite the base 

 of the primitive epithelial fol- 

 licle contributes the tissue of 



the hair-papilla. The region immediately over the papilla is the 

 seat of greatest activity and differentiation : the central cells, con- 

 taining probably many eleidin granules, become converted into the 

 hair and its inner root-sheath, while the peripheral cells of the 



Section of skin of foetal kitten, exhibiting hairs in 

 various stages of development : , superficial layer of 

 epidermis : />, stratum Malpighii from which rudimen- 

 tary hair-follicles extend into connective tissue (c) of 

 primitive corium ; d, e, f, hairs in different stages of 

 their development ; g, sebaceous glands growing from 

 young hair-follicle. 



