THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 447 



After its first formation, the nail is covered by the stratum corneum and 

 the epitrichium, the two together forming the eponychium. The epitrichium 

 soon disappears; later the stratum corneum also disappears with the exception 

 of a narrow band along the base of the nail. 



The formation of nail substance begins during the third or fourth month in 

 the proximal part of the nail area. The nail grows from the root and from the 

 under surface in the region marked by the whitish color (the lunula). New 

 keratinized cells are added from the subjacent stratum germinativum and be- 

 come degenerated to form new nail substance which takes the place of the old as 

 the latter grows distally. 



The Hair. 



The hairs, like the nails, are derivatives of the epidermal layer of the ecto- 

 derm. In embryos of about three months, local thickenings of the epidermis 

 appear (beginning in the region of the forehead and eye-brows) and grow 

 obliquely into the underlying dermis in the form of solid buds the hair germs 

 (Fig. 393, I, II). As the buds continue to elongate they become club-shaped 

 and the epithelium at the end of each molds itself over a little portion of the 

 dermis in which the cells have become more numerous and which is known as 

 the hair papilla (Fig. 392). 



As the epidermal bud grows deeper, its central cells become spindle-shaped 

 and undergo keratinization to form the beginning of the hair shaft; the peripheral 

 layers constitute the anlage of the root sheath (Fig. 393, III, IV). The hair 

 shaft grows from its basal end, new keratinized cells being added from the 

 epithelium nearest the papilla as the older cells are pushed toward the surface 

 of the skin. The surface cells of the hair shaft become flattened to form the 

 cuticle of the hair (Fig. 393, V). The hairs appear above the surface about the 

 fifth month. Of the cells of the root sheath, those nearest the hair become 

 scale-like to form the cuticle of the root sheath; the next few layers become 

 modified (keratinized) to form Huxley's and Henle's layers. Outside of these 

 is the stratum germinativum, the basal layer of which is composed of columnar 

 cells resting upon a distinct basement membrane. The stratum germinativum 

 is continued over the tip of the papilla, where its cells give rise to new cells for 

 the hair shaft (Fig. 393, V). 



The connective tissue around the root sheath becomes differentiated into an 

 inner highly vascular layer, the fiber's of which run circularly, and an outer 

 layer, the fibers of which extend along the sheath. The two layers together con- 

 stitute the connective tissue follicle. 



The first formed hairs, which are exceedingly fine and silky, develop in vast 

 numbers over the surface of the embryonic body and are known collectively as 

 the lanugo. This growth is lost (beginning before birth and continuing during 

 29 



