STRUCTURE OF THE SKIN. 201 



contain merely the terminal loops of the nutritive blood- 

 vessels of the skin (cd). All these different parts of the 

 leather-skin originate, bv differentiation, from the cells, orio*i- 

 nally homogeneous, of the leather-plate, the outer lamella 

 of the skin-fibrous layer (Fig. 112, hpr, vol. i. p. 352; Plates IV. 

 and V., I; Figs. 65-G9, hf, p. 277). 166 



Analogously, all the constituent parts and appendages of 

 the outer-skin (epidermis) originate, by differentiation, from 

 the homogeneous cells of the horn- plate (Fig. 213). At a 



Fig. 213. — Cells of the outer-skin (epidermis) of 

 a human embryo of two months. (After Koelliker.) 



very early period, the simple cell-layer 

 of this horn-plate splits into two dis- 

 tinct strata. The inner, softer stratum 

 (Fig. 212, b) is called the mucous layer; 

 the outer, harder stratum (a), the horn-layer of the outer- 

 skin. The surface of this horn-layer is continually worn 

 out and thrown off; new cell-strata, produced by the 

 growth of the underlying mucous layer, take its place. 

 Originally the outer-skin forms an entirely simple cover 

 over the surface of the body. Afterwards, however, sundry 

 appendages develop from this both internally and ex- 

 ternally. The internal appendages are the skin-glands ; 

 the sweat-glands, the sebaceous glands, etc. The external 

 appendages are hair, nails, etc. 



The glands of the skin-covering are at first merely 

 solid plug-shaped growths of the outer-skin (epidermis), 

 which penetrate into the underlying leather-skin (corium) 

 (Fig. 214 x ). A canal afterwards forms inside these solid 



