THE GERM-PLASM 49 



hardly come into account. We are still far from being able to 

 describe in detail the entire histological process of the differen- 

 tiation of a cell. The passage of invisible ' biophors ' through 

 the pores of the nuclear membrane is probably just as admis- 

 sible an assumption as that of the independent power of motion 

 thereby necessitated in these bearers of vitality ; but the histo- 

 logical structure of a cell is not completed by the mere emission 

 into the cell-body of a few kinds of biophors with great powers 

 of multiplication. Numerous questions suggest themselves in 

 this connection, all pointing to the fact that forces are at work 

 of which w'e are at present ignorant. The immigrating biophors 

 are the mere material which forms the histological structure of 

 a cell, only when subjected to the guiding forces — presumably 

 those of attraction and repulsion — which must be located in the 

 biophor. 



We can as yet form no more exact conception of this process 

 than we can of the manner in which the biophors already con- 

 tained in the cell-body behave in respect to those which have 

 migrated into it from the nucleus. Presumably a struggle of 

 the parts occurs, in which the weaker are suppressed and serve 

 as nutritive material for the stronger ones. But although much 

 remains to be decided by future investigation, the main point at 

 issue, at any rate, viz., that the nature of the cell is really decided 

 by the elements of the nucleus, is definitely established. By the 

 nature of the cell must be understood not only the histological 

 structure of the cell as a whole and^it^ mode of reacting to 

 external influences, but more particularly its mode of division in 

 respect of time and place. It is true that the ctW-body itself and 

 its apparatus for division (the centrosome) primarily determine 

 whether a cell is to divide sooner or later, and into equal or 

 unequal parts ; but these processes always depend finally on the 

 nucleus, which controls the cell-body and impresses on the latter 

 its definite nature. 



The most plausible objection which can be urged against the 

 migration of the particles of the idioplasm into the cell-body is 

 that the substance of the latter is chemically quite different from 

 that of the nucleus. Their behaviour as regards taking up 

 colouring matters is certainly different, as the terms chromosome 

 and chromatin indicate ; but even if a difference in their chemical 

 composition could be inferred from this fact, it would still fail to 

 constitute a decisive proof against the hypothesis of migration : 



