98 THE GERM-PLASM 



— as are those cells which form the first rudiment of the embry- 

 onic integument, — must nevertheless possess several kinds of 

 determinants. We can hardly venture to say whether the three 

 kinds of determinants with which we are here concerned are all 

 present together in the formative cells, and only become distrib- 

 uted amongst special cells when regeneration sets in, or whether 

 they are distributed amongst special cells from the first. Either 

 arrangement is possible. Hence we may assume that some of 

 the young formative cells contain determinants for the glands, 

 and others those for horny or sensory cells, and that the propor- 

 tional numbers and topographical arrangement of these are defi- 

 nitely fixed from the first. A precisely similar assumption is 

 also necessary in the case of embryogeny. 



If, for instance, the sensory organs of the lateral line in a 

 fish or amphibian occur only along the lateral lines and their 

 branches, we must suppose that the subdivision of the idioplasm 

 of the ectodermic cells occurs during the development of the 

 epidermis in such a way that the cells containing the determi- 

 nants of these sensory organs come to be situated only along the 

 lateral lines, and only in definite places on these lines. If now, 

 all the formative cells of the sensory organs do not undergo 

 further development at once, but some of them, on the contrary, 

 remain undeveloped in the immediate neighbourhood — i.e., in 

 the deep layer of young cells — until a necessity for regeneration 

 arises, we can understand in principle why a similar topographi- 

 cal arrangement and numerical relation of the sensory organs to 

 the remaining epidermic elements occurs in the case of regenera- 

 tion, as well as in that of the primary formation of the epidermis 

 in the embryo. 



The idioplasm of the cells does not alone decide what will 

 happen in regenerative processes of this kind. This is shown 

 by the fact that the occurrence of regenerative cell-multiplication 

 depends on a loss of substance, and that the cells cease to pro- 

 liferate as soon as the defect is made good. The stimulus to 

 the further proliferation of the cells ceases at the same time. 

 These facts, however, only give us a very vague insight into the 

 causes of the limitation of the regenerative process ; and w'e 

 shall presently see that the above explanation is insufficient in 

 more complicated cases of regeneration, and that we must, in- 

 deed, assume in addition the existence of other regulating fac- 

 tors, which are situated within the active cells, and not outside 

 them. 



