DEVELOPMENT OF SKIN AND APPENDAGES. 



1403 



FIG. 1169. 



accompanied by the production of a club-shaped enlargement of the hair, between which and 



the shrunken matrix a strand of atrophic epithelial cells for a time remains. With the continued 



progress of these changes, the root of the c/nh-/iair, as the degenerating hair is termed, shortens 



so that the bulbus enlargement recedes from the bottom of the 



hair-sac, until it lies just below the narrow neck of the follicle, 



where it remains for a longer or shorter period until the hair 



is dislodged and finally discarded. A hair that has fallen out 



in consequence of these atrophic changes presents well-marked 



differences in the appearance and structure of its root from a 



growing hair removed by force. In the discarded hair the root 



possesses the characteristic club shape, with contours broken by 



irregular processes composed of the splintered cortical substance, 



which alone forms the terminal bulb that is always solid and has 



neither cuticle nor medulla. 



While the old hair is still lodged in the upper part of the 

 follicle, the first steps towards its replacement are initiated by the 

 stratum germinativum of the old hair-sac. Whether surrounding 

 a new papilla, as held by many, or capping the revived original 

 one (Brunn), the deepest follicle-cells contribute by proliferation 

 the material from which the new hair is developed in a manner 

 agreeing essentially with that in which its predecessor was evolved. 



Papilla 



section of foetal skin, show- 

 e h b a 1r- e fom c fe lan x ^ vel ping 



FIG. 1170. 



The Nails. The first appearance of a definite naii- 

 area on the dorsum of the distal phalanx is seen towards the 

 end of the third foetal month (Kolliker), although Zander 

 has described a local thickening of the epidermis covering 

 the tip of the digit at the ninth week. By the fourth month the nail-area shows 

 as a slightly depressed field that is defined proximally and laterally by a curved 

 swelling, the earliest suggestion of the nail-wall. Distally the field is limited by a 

 transverse elevation. Shortly after the nail-area has been thus defined, the outer 

 cells of its stratum germinativum exhibit deposits of keratohyalin which, by the end 

 of the fourth month, lead to the formation of a thin overlying layer of nail-substance. 

 For a time this gains in thickness by additions to its under surface alone, the primary 

 nail being produced by the progressive conversion of the cells of the stratum granu- 

 losum, which is present throughout the nail-area. 



At this stage the young nail lies completely buried within the epidermis, lying 

 between the most superficial elements of the epidermis and the epitrichial cells above, 

 and the deeper layers of the cuticle below. The overlying epithelial mass, composed 

 of the epidermal and epitrichial elements, constitutes the eponychium, the remains 



of which, after the disappearance of its middle and 

 distal parts, are subsequently seen as a thin mem- 

 brane covering the proximal part of the nail-plate. 

 As yet the young nail-plate has not come into 

 relation with the epidermis of the nail-groove, since 

 it is still confined to the primitive area. But during 

 the fifth month the proximally growing root invades 

 more and more the sulcus until it attains its definite 

 relations with the nail-wall. Meanwhile the nail-bed 

 beneath the developing root undergoes thickening 

 and becomes the matrix, while the cells containing 

 keratohyalin gradually disappear from the distal 

 region of the nail-area in consequence of their com- 

 pleted conversion into the nail-substance. Subse- 

 quently these cells are limited to the proximal 

 nail-producing zone of the matrix from which, after 

 the initial formation of the primary nail-substance, 

 the nail alone receives the additions necessary for its 

 continued growth. In consequence of the resulting 

 forward growth the nail pushes its way through the elevated distal boundary of the 

 nail-field, the epithelium lying above the nail-plate being lost, while that below remains 

 as the representative of the sole-plates that are well marked in many other animals. 



Section of foetal skin, showing develop- 

 ing sweat-glands ; a, is less advanced than 

 6 and c. X 100. 



