244 TEE POPULAR SCIENCE MONTHLY 



substances or parts originally present and by their reactions to external 

 stimuli new substances and parts appear which had no previous exist- 

 ence, just as new substances result from chemical reactions. This is 

 " creative synthesis " in philosophy, epigenesis in development. Differ- 

 entiations appear chiefly in the cytoplasm, but only as the result of inter- 

 action between cytoplasm and nucleus. Similarly, it may be argued, 

 smaller units of organization, such as chromosomes or chromosomeres, do 

 not in themselves give rise to any adult part, but only as they interact 

 upon one another are new parts formed. 



In many cases the first formation of such new substances appears in 

 the immediate vicinity of the nucleus and, like assimilation itself, is 

 evidently brought about by the interaction of nucleus and cytoplasm. 

 In certain cases it can be seen that the achromatin and oxychromatin 

 which escape from the nucleus during division take part in the forma- 

 tion of new substances in the cell body, and since the oxychromatin is 

 derived from the chromosomes of the previous cell division, it is probable 

 that the chromosomes are a factor in this process. 



Weismann maintained that the chromosomes and the inheritance 

 units contained in them undergo differentiation by a process of disin- 

 tegration and that these disintegrated units escape into the cell body 

 and there produce different kinds of cytoplasm in different cells. A 

 somewhat similar view was advanced by deVries in his theory of intra- 

 cellular pangenesis. However, as we have seen already, there is good 

 evidence that the chromosomes do not undergo progressive differentia- 

 tion in the course of development; they always divide with exact 

 equality, and even in highly differentiated tissue cells their number and 

 form remain as in embryonic cells. 



On the other hand, the cytoplasm undergoes progressive differentia- 

 tion, and when by pressure or centrifugal force such differentiated cyto- 

 plasm is brought into relations with strange nuclei the differentiations 

 of the cytoplasm are not altered thereby, thus showing that the dif- 

 ferent nuclei are essentially alike and that differentiations are mainly 

 limited 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 

 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 are necessary to the formation of 

 the new cytoplasmic substances which appear in the course of devel- 

 opment. 



2. Segregation and Isolation of Different Substances in Cells 



But differentiation consists not only in the formation of different 

 kinds of substances in cells, but also in the separation of these substances 

 from one another. This separation is brought about to a great extent 



