The Cellular Basis 205 



tures which are characteristic of the different kinds of tissue cells. 

 We shall here describe only the first and second of these pro- 

 cesses which are of more general interest than the last. 



i. The Formation of Different Substances in Cells. Embry- 

 onic differentiation consists primarily in the formation of dif- 

 ferent kinds of protoplasm out of the protoplasm of the germ 

 cells. It is plain that different kinds of protoplasm are present 

 in the two germ cells before they unite in fertilization, but in the 

 course of development the number of substances present and the 

 degree of difference among them greatly increase. 



Actual observation shows that by the interaction with one 

 another of substances or parts originally present and by their 

 reactions to external stimuli new substances and parts appear 

 which had no previous existence just as new substances result 

 from chemical reactions. This is "creative synthesis" in general 

 science, epigenesis in development. Differentiations appear 

 chiefly in the cytoplasm but only as the result of interaction be- 

 tween cytoplasm and nucleus. Similarly, it may be argued, smal- 

 ler units of organization such as chromosomes or chromomeres 

 do not in themselves give rise to any adult parts, but only as they 

 interact upon other units are new parts formed. 



In many cases the first formation of such new substances ap- 

 pears in the immediate vicinity of the nucleus and, like assimila- 

 tion itself, this is evidently brought about by the interaction of nu- 

 cleus 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 formation 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 inheri- 

 tance units contained in them undergo differentiation by a pro- 

 cess of disintegration and that these disintegrated units escape 

 into the cell body and there produce different kinds of cytoplasm 



