148 SPECIAL HISTOLOGY. 



timies to increase in thickness, in consequence of the multipli- 

 cation of its elements, whilst the horny layer, for the increase 

 of its proper substance and to replace what it loses by des- 

 quamation, recruits itself from it exactly as in the adult. How 

 the multiplication of the cells goes on in the rete Malpighii 

 has not been directly observed ; but it is certainly not by 

 free cell-development, since in embryos of all ages the mucous 

 layer consists wholly of cells, and free nuclei are altogether 

 absent. As regards the horizontal extension of the epidermis, 

 it appears, as Harting (' Rech. micrometr/ p. 47) justly observes, 

 from the circumstance that the epidermic scales of the foetus 

 and of the adult differ very little in superficial size, that it 

 can only in a very slight degree be ascribed to the growth 

 of its elements. In fact the horny plates of the embryo of 

 fifteen weeks already measure 0-009 — 0012'", in the sixth 

 month 001 — 0-012'", in the seventh month 001— 0-014'", 

 in the new-born infant 0-012 — 0-016'", in the adult 0-008 — 

 0016"'. Since, however, keeping in mind the structure of 

 the horny layer, it cannot well be supposed that new scales 

 are continually intercalated from below between its elements, 

 and since a superficial multiplication of the cells of the rete, 

 which also do not increase in size, must certainly be granted, 

 it seems impossible to admit any other conclusion than that, 

 in agreement with the great superficial growth of the cutis 

 and of the rete, and the small extensibility of the horny 

 layers, a series of desquamations of the latter take place, 

 Avhich, if this view be correct, must likewise obtain after birth. 



[In an embryo of five weeks I found, in the place of the 

 epidermis, nothing but an external layer of polygonal cells of 

 0012 — 0-02'" in diameter, and an internal laver of small cells 

 of 0-003 — 0004'"; in embryos of fifteen weeks the epidermis 

 is 0*01 — 0-012'", thick and composed as before, only that 

 the deep layer of cells answering to the stratum mucosum is 

 often already double, and the external cells measure only 

 0-009— 0-012'". 



In the fifth month the epidermis in one instance measured 

 on the heel and ball of the hand 0-02 — 0024'", over the 

 ridges of the cutis 036 — 0-04'", in the furrows between them 

 on the back 002 — 0-024'", of which thickness one third may 



