198 Heredity and Environment 



Share of Chromosomes and Cytoplasm in Heredity and Devel- 

 opment. — It will be observed that the correlation between chromo- 

 somes and adult characters is different in kind from that between 

 the cytoplasm of the egg and adult characters; in the latter case 

 polarity, symmetry and pattern are characters of the same kind 

 in the egg and in the adult, and the correspondence is compara- 

 tively close ; in the former there is no correspondence in kind 

 between the chromosomal peculiarities and the peculiarities of 

 the adult. This fact suggests that the chromosomal organization 

 is more fundamental than that of the cytoplasm ; the chromo- 

 somes contain the germplasm, the cytoplasm is the somatoplasm ; 

 the chromosomes are chiefly concerned in heredity, the cytoplasm 

 in development. 



E. THE MECHANISM OF DEVELOPMENT 



Development consists in the transformation of the oosperm into 

 the adult. What is the mechanism by which this transformation 

 is effected ? There is progressive differentiation of the germ into 

 the developed organism but by what process is this differentiation 

 accomplished ? 



Many different processes are concerned in embryonic differ- 

 entiation. From the standpoint of the cell the most important of 

 these are (1) the formation of different kinds of substances 

 in cells, (2) the localization and isolation of these substances, 

 (3) the transformation of these substances into various struc- 

 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. 



1. 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. 



