THE CELLULAR BASIS OF HEREDITY 243 



duced eggs of the vertebrate pattern ; but the color of our skin and hair 

 and eyes, our sex, stature, and mental peculiarities were determined by 

 the sperm as well as by the egg from which we came. There is evi- 

 dence that the chromosomes of the egg and sperm are the seat of the 

 differential factors or determiners for Mendelian characters, while the 

 general polarity, symmetry and pattern of the embryo are determined 

 by the cytoplasm of the egg. 



It will be observed that the correlation between chromosomes and 

 adult characters is different in kind from that between the cytoplasm 

 of the egg and the adult characters ; in the latter case polarity, symmetry 

 and pattern are of the same kind in the egg and in the adult, and the 

 correspondence is comparatively close; in the latter there is no cor- 

 respondence in kind between the chromosomal peculiarities and the 

 peculiarities of the adult. This fact might suggest that the chromo- 

 somal organization may be more fundamental than that of the cyto- 

 plasm. There are reasons for believing that many substances of the 

 cell are formed by the interaction of nucleus and cytoplasm, and most 

 probably the chromosomes are an important factor in this process. But 

 in no case is the cytoplasm a negligible factor — in no case does it serve 

 merely as food for the chromosomes. The entire cell, nucleus and 

 cytoplasm, is concerned in heredity and differentiation. 



D. 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 ac- 

 complished ? 



Many different processes are concerned in embryonic differentiation. 

 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 sub- 

 stances into the various structures which are characteristic of the dif- 

 ferent kinds of tissue cells. We shall here describe only the first and 

 second of these processes which are of more general interest than 

 the last. 



1. The Formation of Different Substances in Cells 



Differentiation consists primarily in the formation of different kinds 

 of protoplasm from the proptoplasm 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 these substances and the degree of difference between them greatly 

 increase. 



Actual observation shows that by the interaction with one another of 



