HEREDITY AND EVOLUTION 105 



mation and action of the karyokinetic figure con- 

 stitute a puzzle for which, as it seems to me, no 

 adequate solution has yet been found. But the 

 effect of its action gives us good reason to regard 

 it as the most important instrument by which the 

 nuclear substance is handed on with its integrity 

 unimpaired from generation to generation of 

 cells. 



DIFFERENTIATION 



Our third question involves the problem of dif- 

 ferentiation, which is inseparable from that^of 

 cell-metaboUsm in general, since it involves the 

 mode of interaction of nucleus and protoplasm. 

 It is a significant fact that visible structural dif- 

 ferentiation affects the protoplasmic substance in 

 far greater degree than the nuclear. Both in 

 their structure and in their modes of activity the 

 most important characteristics of different kinds 

 of cells are found in the protoplasm. To some 

 extent, no doubt, the nuclei of different tissues . 

 show certain characteristic pecuharities, and these 

 can in a measure be correlated with the function 

 of the cells. It is nevertheless obvious that the 

 most characteristic features of the muscle-cell, 

 the nerve-cell, or the gland-cell are displayed in 

 the protoplasmic rather than the nuclear sub-' 

 stances. And this again falls into hne with the 

 view that the nucleus is the main conservative 

 element of the cell-system, the protoplasm the 

 plastic element through the modifications of 



