THE GEKM-PLASM THEORY 381 



number and arrangement, and on the size and on the frequency of 

 repetition of certain parts. These distinctive characters are just 

 as constant and as strictly transmissible, and may be as heritably 

 variable as those which depend on specific cell-differentiation, and 

 they must therefore likewise be determinable by definite elements 

 of the germ-plasm. Obviously enough, however, these elements are 

 not of the same nature as the known specific histological elementary 

 particles; they can be neither nerve-, muscle-, nor gland-biophors. 

 They must rather be vital units of such a kind that they communicate | 

 to the cells and lineage of cells, into whose bodies they migrate from j; 

 within the nucleus, a definite vital power, that is, an organization!: 

 which regulates the size, form, number of divisions, and so on, of 

 these cells — in short their whole prospective significance. Always, 

 however, they act in co-operation with the cell-body into which theyj 

 have penetrated. 



Throughout we must hold ourselves aloof from the idea that 

 ' characters ' are transmissible. It is customary, indeed, to speak 

 as if this were so, and it is also necessary, because we can only 

 recognize the ' characters ' of a body, and not the essential ' nature ' 

 on which these characters depend ; but the determinants are not seed- 

 grains of individual characters, but co-determinants of the nature 

 of the parts which they influence. There are not special determinants 

 of the size of a cell, others of its specific histological differentiation, 

 and still others of its duration of life, power of multiplication, and 

 so on ; there are only determinants of the whole physiological nature 

 of a cell, on which all these and many other 'characters' depend. 

 For this reason alone I should object to the assumption that thei 

 determinants of the germ are ready-made histological substances, j 

 That is as unlikely as that their groups in the germ-plasm are 

 'miniature models' of the finished parts of the body. 



I conceive of the process of cell-differentiation as follows: at 

 every cell-stage in the ontogeny determinants attain to maturity, and 

 break up so that their biophors can migrate into the cell-bodies, 

 so that the quality of each cell is thus kept continually under control, 

 and may be more or less modified, or may remain the same. By the 

 'maturity' of a determinant I mean its condition when by continual 

 division it has increased in number to such a point that its disintegration 

 into biophors and their migration into the cell-substance can take place. 



One more point I must touch upon here, the question of the 

 'liberation' or 'stimulation' of the determinants. The activity of 

 an organ never depends on itself alone ; the contraction of a muscle is 

 induced by a nerve stimulus or by an electric current; the activity 



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