154 SPECIAL HISTOLOGY. 



4 



the deepest round cells, or a multiplication by division, for which 

 latter view the occasional occurrence of two nuclei in some of the softer 

 epidermic cells, seems to speak. It can be more easily made out, how, 

 in the course of the growth of the epidermis, the youngest epidermic 

 cells become changed into horny plates. The small and round vesicles 

 of the deeper layers of the stratum 3Ialpigldi become larger and flatter 

 the more they approach the surface, until at last they are completely 

 converted into flattened plates. In the meanwhile their nuclei at first 

 grow a little, and then, as a general rule, disappear wholly in the horny 

 layers ; whilst the cell-contents, which are granular in the mucous layer, 

 clearly distinct from the cell-membrane, and probably semi-fluid, become 

 more solid and homogeneous in the horny layer, and finally coalesce with 

 the cell-membranes. At the same time the latter are chemically altered, 

 becoming less and less soluble in caustic alkalies. 



§ 47. Development of the Epidermis. — The first layers of the epi- 

 dermis are developed, in the Mammalia, by the metamorphosis of the 

 most superficial of the original formative cells which compose the young 

 embryo. When the rudiments of the stratum 3Ialpighii and horny layer 

 are once indicated, the former continues to increase in thickness, in con- 

 sequence of the multiplication of its elements, whilst the horny layer, 

 for the increase of its proper substance and to replace what it loses by 

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

 multiplication of the cells goes on in the rete Malpigliii 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. microm^tr," p. 47) justly ob- 

 serves, 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 

 -0-012, in the sixth month 0-01-0-012, in the seventh month 0-01- 

 0-014, in the new-born infant 0-012-0-016, in the adult 0-008-0-016 

 of a line. Since, however, keeping in mind the structure of the horny 

 layer, it cannot well be supposed that new scales are continually inter- 

 calated from below between its elements, and since a superficial multi- 

 plication 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, which, if this view be correct, 

 must likewise obtain after birth. 



