62 LECTURE n 



the epidermis with regard to one another. The extremely 

 dense and hard scales, which constitute the uppermost 

 part, the so-called body of the nail (Nagelblatt), may, by 

 different methods, be restored to forms in which they 

 present the ordinary appearances of cells , arid this is 

 best seen after treatment with an alkali, when every 

 scale swells up into a large, broadly oval, cell. 



In the uppermost layers of the epidermis the cells be- 

 come everywhere flatter, and towards the external surface 

 no more nuclei at all can be discovered in them. Still 

 there is no original difference between the epidermis and 

 the rete Malpighii ; the latter is only the matrix of the 

 epidermis, or indeed its youngest layer, inasmuch as 

 from it there is a constant apposition of new parts tak- 

 ing place, which gradually become flatter and flatter, and 

 move upward as fast as the scales on the outside disap- 

 pear through friction of the surface, washing, or rubbing. 

 But between the lowest layer of the rete and the sur- 

 face of the cutis vera there are no intervening layers ; 

 there is no amorphous fluid or blastema to be found 

 there in which the cells could be generated, but they lie 

 in direct contact with the papillae of connective tissue of 

 the cutis. There is therefore nowhere any space here, 

 as there was thought to be even a short time ago, into 

 which fluid transudes from the papillae and the vessels 

 contained therein, in order that new cells may arise and 

 develop themselves out of it. Of such a fluid there is 

 absolutely nothing discernible, but throughout the whole 

 series of the layers of cells of the rete and epidermis the 

 same relations exist that we are familiar with in the 

 bark of a tree. The cortical layer of a potato (Fig. 7) 

 exhibits in a similar manner, externally, corky, epider- 

 moidal cells, and underneath, as in the rete Malpighii, a 

 layer of nucleated cells, the cambium, constituting the 

 matrix for the subsequent growth of the cortex. 



