THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 439 



The Nails. 



The nails are derivatives of the epidermal layer of the ectoderm, and cor- 

 respond morphologically to the claws and hoofs of lower animals. The 

 epidermis on the end of each finger and toe forms a thickening, known as the 

 primitive nail, which is encircled by a faint groove (Zander). This occurs 

 about the ninth week. Later the nail area migrates to the dorsal side of the 

 digit and becomes somewhat sunken below the surface of the surrounding 

 epithelium (Fig. 391). These observations have led to the conclusion that 

 primarily the nails in man occupied positions on the ends of the digits, cor- 

 responding to the positions of the claws in low r er forms. Furthermore, the fact 

 that the nails (or their anlagen) are at first situated on the ends of the digits and 

 subsequently migrate dorsally would exolain the innervation of the nail region 

 by the palmar (and plantar) nerves. 



Strat. corneum "I 



1 Epidermis 



#> " I Strat. germinativum J 



/.-*I^ 



V-V **: 



*i I SfSP@f- HairBerm 



(*' Hf Hair papa 





papilla 



_- i--,.^ , w.w - , 



Con. tis. follicle 



Hair germ 



Hair papilla Connective tissue 



follicle 



FIG. 392. Vertical section of the skin of a mouse embryo of 18 mm., showing 

 early hair germs. Maurer. 



After the dorsal migration of the nail area, the epithelium and dermis along 

 the proximal and lateral edges become still more elevated to form the nail wall, 

 the furrow between the latter and the nail being the nail groove. At the distal 

 edge of the nail area, the epithelium becomes thickened to form the so-called 

 sole plate, which is probably homologous with the more highly developed sole 

 plate in animals with hoofs or claws. The epithelium of the nail area increases 

 in thickness, and, as in the skin, becomes differentiated into three layers 

 (Fig. 391). The outer layers of cells become transformed into the stratum 

 corneum. The cells of the next deeper layers, which acquire keratin granules 

 and constitute the stratum lucidum, degenerate and give rise to the nail sub- 

 stance. Thus the nail is a modified portion of the stratum lucidum. The 

 layers of epithelium beneath the nail form the stratum germinativum, which, 

 with the subjacent dermis, is thrown into longitudinal ridges. 



