GENETIC SYSTEM: RELATION TO CHARACTERISTICS 103 



We know further that they play a most important part in 

 determining sex, with all that this includes of structural 

 and physiological influence (Chapter III). 



Clearly therefore the X-chromosomes play a most im- 

 portant part in development. Their action is not limited to 

 any single part of the body, nor to any single class of func- 

 tions. They enter into the processes of development in such 

 a way as to influence all parts of the body, all functions of 

 the body. They begin to influence development very early, 

 as was seen in considering the development of sex. And 

 they play important roles in such relatively late processes as 

 the production of eye colors. It can hardly be doubted that 

 they enter into the developmental activities from practically 

 the beginning, influencing all the later processes that occur. 



NOTES AND REFERENCES ON CHAPTER IV 



1. Page 91. See L. V. Morgan (1922), Non-criss-cross Inheritance 

 in Drosophila melanogaster. Biological Bulletin, vol. 42, pp. 267-273. 



2. Page 98. For inheritance of characteristics in man, see R. R. Gates 

 (1929), Heredity in Man. 385 pages, London; also E. Baur, E. 

 Fischer and F. Lenz (1931), Human Heredity. 734 pages, New York. 



