CHARACTERISTICS: RELATION TO ENVIRONMENT 237 



is these diverse chemical products that cause the bodies 

 of the two sexes to develop differently. In the embryo con- 

 taining but one X-chromosome a certain kind of hormone 

 is produced, in that having two X-chromosomes another 

 sort of hormone. These two diverse hormones then circu- 

 late through the two bodies; they act diversely in the two, 

 causing one to produce the characteristics of the male sex, 

 the other the characteristics of the female sex. Body cells 

 of either type (as set forth in Chapter III) may produce 

 either type of characteristics, depending on the hormone 

 that acts upon them. That is, what their genes produce 

 depends on the chemical conditions that surround them. Di- 

 versity of sex, like diversity in tissue differentiation, is pro- 

 duced through adjustment of the cells to their environ- 

 ment; in this case to hormonal environment. 



It has been discovered that many other diversities 

 between individuals, and between parts of the body, are 

 produced by other kinds of hormones, by hormones se- 

 creted by the thyroid gland, by those from the hypophysis, 

 and from other parts. Details as to these cannot be taken 

 up here. What they agree in showing is this: The genes 

 first act to produce different chemicals. Then these diverse 

 chemicals cause the cells to develop in certain particular 

 ways, so as to give rise to diverse parts. By this continued 

 interaction of genes with cytoplasm, of cells with each 

 other, of cells with their surroundings, there is finally pro- 

 duced the entire body with all its parts and functions. 



Thus in the process of development, adjustment of the 

 cells to their environment, response to the surrounding 

 conditions, play a fundamental part. The genes show 

 themselves most sensitive to the conditions surrounding 

 them. 



Since genes act by producing hormones, and these hor- 

 mones then determine many features of the later develop- 

 ment, the question arises as to whether such hormones 



