REGENERATION 



95 



a stock in which the necessary substitutes for the older cells can 

 constantly be produced. The whole supply of the corresponding^ 

 determinants is not therefore removed from the body simul- 

 taneously by the loss of the worn-out cells, for the young cells 

 which remain contain determinants of the same kind. In the 

 human epidermis, this stock of young cells constitutes the so- 

 called ' rete Malpighii ' or ' mucous layer/ in which new cells are 

 constantly being formed by division ; these, in proportion as 

 they become older, are gradually pushed upwards mechanically 

 from the deeper into the superficial layers, while the deepest 

 layer of all consists entirely of young cells which are capable of 

 division. 



No special theoretical assumption need be made to explain this 

 process. We must only suppose that the first formed epidermic 

 cells are endowed in advance with a capacity for reproduction 

 during many generations. It must be assumed that the repro- 

 ductive power of a cell is regulated by the idioplasm, because 

 the power and rate of multiplication are essential qualities of a 

 cell, and, as we have seen, are controlled by the nuclear substance. 

 But we cannot at present even form a conjecture as to which 

 qualities of the idioplasm the degree and rate of the capac- 

 ity for reproduction are due. We must be satisfied with 

 attributing to the cells which form the epidermis of the embryo 

 an idioplasm possessing a definite reproductive power, which 

 gradually decreases. We can further only suppose that the 

 idioplasm retains its constitution during life, or, in other 

 words, that the determinant of a particular part of the epidermis 

 is always retained in the permanent stock of young cells. 

 Regeneration depends simply on a regular increase of those cells 

 which contain epidermic idioplasm. 



The nature of the epidermis is not the same in different parts 

 of the human skin : thus it differs on the volar and on the dorsal 

 surfaces of the fingers ; and, again, on the two basal and on the 

 ungual segments. But this fact does not stand in the way of 

 the theoretical explanation of regeneration, for the determinants 

 of different parts must differ som.ewhat from one another. Even 

 in places where two or more dissimilar parts are situated close 

 together, the retention of the limits between them, during their 

 continual regeneration, may be explained simply by the fact that 

 the different regions of the tissue are regenerated by formative 

 cells possessing different determinants. 



