298 



HISTOGENESIS 



THE NAILS 



The anlages of the nails proper are derived from the epidermis and 

 may be recognized in fetuses of 45 mm. (CR). A nail anlage forms on 

 the dorsum of each digit and extends from the tip of the digit almost to 

 the articulation of the terminal phalanx. At the base of the anlage, that 

 is, proximally, the epidermis is folded inward to form the proximal nail 

 fold (posterior nail fold of the adult) (Fig. 303 C). The nail fold also ex- 

 tends laterally on either side of the nail anlage and forms the lateral nail 

 fold of the adult (A, B}. 



The material of the nail is developed in the lower layer of the proxi- 

 mal nail fold (C). In certain of 'the epidermal cells, which, according to 



Sole plate 



Eponychium 



FIG. 303. Figures showing the development of the nail. A, From a 40 mm. human fetus 

 (X 20); B, from a 100 mm. fetus (X 13); C, longitudinal section from a 100 mm. fetus (X 24). 

 (Kollmann.) 



Bowen, represent a modified stratum lucidum, there are developed kera- 

 tin, or horn fibrils during the fifth month of fetal life. These appear 

 without the previous formation of keratohyalin granules, as is the case in 

 the cornification of the stratum corneum. The cells flatten and form the 

 plate-like structure of which the solid substance of the nail is composed. 

 Thus the nail substance is formed in the proximal nail fold as far distad as 

 the outer edge of the lunula (the whitish crescent at the base of the adult 

 nail). The underlying epidermis, distal to the lunula, takes no part in the 

 development of the nail substance. The corium throws its surface of 

 contact with the nail into parallel longitudinal folds that produce the 

 longitudinal ridges of the nail. The nail is pushed toward the tip of the 



