236 GENETICS 



are to be produced. If one turns them side for side or end 

 for end, the results are the same. Any cell can produce al- 

 most any of the parts of the body. 



That is, then, from the beginning the cells adjust them- 

 selves to their surroundings, to their environment. What 

 part of the body each cell becomes is determined, not by 

 the genes it contains — for each contains all the genes — but 

 by the conditions surrounding it. The genes within the cell 

 are enormously sensitive to their environment, to the cells 

 surrounding them. They alter their method of action to fit 

 the situation, producing whatever is the next thing in the 

 body pattern. Development is fundamentally adjustment to 

 environment, in this case, to cellular environment. 



Of course what the genes may produce is not without 

 limits. The genes of a frog produce the parts of a frog's 

 body; the genes of a man produce the parts of a man's body. 

 The genes of a Chinese produce the body of a Chinese; the 

 genes of a Caucasian the body of a Caucasian. Some 

 genes produce an individual of light complexion, others 

 individuals of dark complexion. Just what the gene shall 

 produce in development is always dependent both on the 

 nature of the genes that are at work, and on the condi- 

 tions in which they find themselves. And in this they give 

 a true picture of the interactions of heredity and environ- 

 ment. 



The dependence on the conditions surrounding the cells, 

 of what the genes within the cells produce, is shown In cer- 

 tain other ways, in the production of differences of sex, of 

 which some account is given in Chapter III. As there seen, 

 in many organisms the two sexes begin life with different 

 chromosome combinations : one sex has two X-chromo- 

 somes, the other but one. It is to this original difference 

 in gene combination that the later sex difference Is ulti- 

 mately to be referred. But the original difference in chromo- 

 somes acts through intermediate chemical products, and it 



