182 HEREDITY AND ENVIRONMENT 



by deVries in his theory of intra-cellular pan- 

 genesis. However, as we have seen already, 

 there is good evidence that the chromosomes 

 do not undergo progressive differentiation in 

 the course of development; they always divide 

 with exact equality, and even in highly differ- 

 entiated tissue cells their number and form 

 usually remain as in embryonic cells. 



On the other hand the cytoplasm undergoes 

 progressive differentiation, and when by pres- 

 sure or centrifugal force such differentiated 

 cytoplasm is brought into relations with 

 strange nuclei the differentiations of the cyto- 

 plasm are not always altered thereby, thus 

 showing that the different nuclei are essentially 

 alike and that differentiations are mainly lim- 

 ited to the cytoplasm. Thus the differentiations 

 of cells are not due to the differentiations of 

 their nuclei, but rather the reverse is true, such 

 differentiations of nuclei as occur are due to 

 differentiations of the cytoplasm in which they 

 lie. Nevertheless differentiations do not take 

 place in the absence of nuclear material, and it 

 seems probable that the interaction of nucleus 

 and cytoplasm is necessary to the formation of 



