Skin and Its Derivatives 



511 



matoglyphics) in personal identification. 

 The development of the epidermal ridges 

 is intimately associated with the develop- 

 ment of the touch balls or volar pads. These 

 embryonic structures are definitely localized 

 swellings or bulges foimd on the terminal 



but in the deep germinal portion in contact 

 with the mesoderm or corium, and is very 

 likely a response to inductive stimvdi from 

 the mesoderm. The lower germinal layer 

 begins to increase and form folds which 

 grow downwards into the corium. Simul- 



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Fig. 196. A, Section through head skin of newborn black mouse (C57) showing various stages in the 

 formation of hairs. Delafield's hemato.xylin, 10/jl. X 157. B, Section through dorsal skin of black mouse 

 (C57), 7 days after birth. Note fully differentiated, emerged, pigmented hairs. Lightly stained with Dela- 

 field's hematoxylin, lO^u. X 75. 



segments of the digits and on the palms and 

 soles. In the human, the volar pads of the 

 hand are evident early in fetal life (sixth 

 week) when the hand is still paddle-like. 

 They become quite prominent about the 

 twelfth to thirteenth week. Soon afterwards 

 they regress and become relatively incon- 

 spicuous. 



Ridge formation begins when the volar 

 pads are at their peak. The first indication 

 ir, seen, not on the surface of the epidermis, 



taneously, the surface of the corium in con- 

 tact with the epidermis develops elevations 

 or folds (papillae) which project upwards 

 into the epidermis and alternate with the 

 similar downward projections from that 

 layer. Later the outer surface of the epi- 

 dermis, which has up until now remained 

 smooth, becomes raised into ridges, one cor- 

 responding to each of those formed earlier 

 on the lower surface. 



Like feathers and hairs, the epidermal 



