THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 



445 



the deeper layers, constitute the stratum corneum (Fig. 392). In the thick 

 epidermis, on the palms of the hands and the soles of the feet, for example, a 

 few layers of cells just outside of the stratum granulosum become specially 

 modified (keratinized) to form the stratum lucidum. 



THE DERMIS. In the first month the dermis is represented by closely ar- 

 ranged, spindle-shaped mesenchymal (mesodermal) cells underlying the 

 epidermis, and is separated from the latter by a delicate basement membrane. 

 This mesenchymal tissue gives rise to fibrous connective tissue which, about 

 the third month, becomes differentiated into two layers the dermis proper 

 and the deeper subcutaneous tissue. The papillas develop as little projections 

 of the dermis which grow into the stratum germinativum of the epidermis. 

 In some of these, many blood vessels appear, while in others nerve endings 



Root of nail 



Eponychiutn 



Phalanx II 



Sole plate 



Sweat glands 



FIG. 391. Longitudinal section through the end of the middle finger of a 

 5 months human foetus. Bonnet. 



(tactile corpuscles of Meissner) develop, thus giving rise to vascular and nerve 

 papillae. Usually a considerable amount of fat develops in the subcutaneous 

 tissue. Some of the mesenchymal cells of the dermis are transformed into 

 smooth muscle cells which are found in connection with the hairs (arrectores 

 pilorum), in the scrotum (tunica dartos), and in the nipples. 



The dermis has generally been considered as a derivative of the cutis plates 

 (p. 167) which, with the myotomes, constitute the outer walls of the primitive 

 segments, but it is probable that the outer walls of the segments are trans- 

 formed wholly into muscle tissue (McMurrich). 



The pigment in the dermis develops in the form of granules in the connect- 

 ive tissue cells; that in the epidermis appears as granules in the cells of the deeper 

 layers (white races) or of all the layers (dark races). Whether the pigment in 

 the epidermis arises independently or is carried from the dermis by wandering 

 cells is not known. 



