REGENER^TIOX 



113 



those tissues which belong to the germinal layer from which 

 they were developed.'' But this statement can only be true in 

 the case of the highest Vertebrates, for, as the brothers Hertwig 

 have shown, the germinal layers of the Metazoa are not primi- 

 tive organs in the histological sense ; and moreover, in the lower 

 animals, several, if not all of the tissues, can be formed from 

 each of the germinal layers. In lower animals, not only all the 

 varieties of tissue, but under certain circumstances even rows of 

 cells of one primary germinal layer and even indeed the entire 

 animal, may arise from young cells belonging to the other 

 germinal layer. In the chapters on multiplication by fission 

 and gemmation, this process will be traced to its origin in the 

 idioplasm. At present we have only to deal with the question 

 as to whether the determinants of the various kinds of cells 

 which are required for regeneration are contained within young 

 cells only, or whether they are also present in those which have 

 become differentiated histologically. 



Although the supplementary determinants are certainly in 

 many cases contained in young cells without any specially 

 marked histological character, their distribution can neverthe- 

 less hardly be limited to these cells exclusively. It may happen — 

 as will be show-n in greater detail subsequently — that cells, which 

 are fully developed histologically, both in plants and in the 

 lower animals, contain all the determinants of the species ; 

 that is to say, they may contain germ-plasm as supplementary 

 idioplasm. Hence there is no reason to assume that smaller 

 groups of determinants may not have been supplied to specific 

 tissue cells wherever they were required, although I am unable 

 to give a definite example of such a case. 



Although regeneration may originate in most cases in voung, 

 or so-called ' embryonic ' cells, it is nevertheless quite a mistake 

 to connect the idea of the undiiferentiated state of these cells 

 with this fact, as is so often done. These ' embryonic cells' are 

 not ' capable of giving rise to anything and everything,' for each 

 of them can only develop into that kind of cell the determinant 

 of which it contains. Under certain circumstances such a cell 

 may contain several different determinants at the same time, 

 which are only distributed amongst the individual cells in sub- 

 sequent cell-generations ; but the structure which can and will 

 become developed from it always depends on the cell itself, and 

 its fate is determined by the idioplasm it contains, and can only 



