Eye-Color Hormones in Drosophila 307 



produced by the ordinary genes found in the nucleus. Different 

 chemical groups which become available under special conditions 

 then combine with these plasmagenes to modify them. The 

 hapten which modifies a plasmagene may be of the nature of 

 a hormone but may be produced as the result of gene action. 

 AVhether this picture of Wright's is the correct explanation 

 of gene physiology we cannot say at present. All these processes, 

 however, whether gene formation, cellular metabolism, cell 

 growth, or cell differentiation, are based on the belief that genes 

 can synthesize substances like themselves. 



Eye-Color Hormones in Drosophila 



Gene products have been mentioned frequently. The curious 

 reader will surely ask what gene products are and how they 

 act. This question is not easy to answer because, being of an 

 intracellular nature, they do not lend themselves readily to ex- 

 perimentation. It is generally believed that gene products are 

 of the nature of catalysts or hormones, but experimental evi- 

 dence to support this view is not extensive. Occasionally, how- 

 ever, the presence or absence of certain hormones in individuals 

 of a certain genotype has been demonstrated. 



Beadle, Ephrussi, Tatum, and others have given us consid- 

 erable knowledge of the hormones necessary for the develop- 

 ment of the wild-type eye color in Drosophila melanogaster. 

 They found that two water-soluble hormones, v^ and cn"^ , must 

 be present for the development of the particular shade of red 

 found in the eye of the wild-type fly. In the wild type, two pig- 

 ments must be present. One pigment is soluble in water. Its 

 color depends upon the acidity of the solution in which it is 

 found, for it is red in a base and yellow in an acid. Since it is 

 normally red in the wild-type fly, it can be considered a red pig- 

 ment. The other pigment is insoluble in water and is brown, 

 but if it is oxidized it is yellow and if reduced is red. In the 

 wild-type fly both reduced and oxidized forms are present in 

 equilibrium, and the color is brown. 



The exact shade of the eye will depend upon the relative 

 amounts of these two pigments. In the white-eyed fly, neither 

 pigment is present, and the eye is colorless or ''white." If the 

 red pigment alone is absent, the eye is brown. Such an eye 

 color is found in flies homozygous for the recessive gene, hw, but 



