88 



THE REALISATION OF THE NUCLEAR FACTORS 



the v-mutant itself. Similarly, a factor necessary for the forma- 

 tion of brown pigment in cn-eyes is present in the wild form, 

 but not in either v- or cn-mutants. Apparently the cells of 

 certain organs (eyes, Malpighian tubes, fat-body) of the normal 

 animal produce an excess of these substances. Transplantation 

 of these organs from a normal animal into a v- or cn-mutant 

 resulted in normal, wild type coloration of the host's eyes. 

 Injection of lymph from a normal animal had the same effect. 

 Therefore, the substances concerned must be present in lymph 

 as well. Further experiments showed that in a normal, wild 

 type Drosophila a reaction-chain occurs which first leads to 

 the production, under the influence of a single gene, of a so- 

 called v-h substance. Under the influence of a second gene this 

 substance is transformed into a second substance, the cn-\- 

 substance, which in its turn is indispensable for the normal 

 pigmentation of the eyes. In the mutants, this catenary reaction 

 is interrupted in various places. In "vermilion", the gene that 



ye/Z/cA 



}/^C/7 



Fig. 31. Drosophila. (a) the fat body of a normal animal {v^rcn-\-) 

 is transplanted into a vcn-\- animal, where it secretes v-\- substance; 

 (b) the host tissues transform this into cn-\- substance, so that both 

 the eyes of the host, and a grafted v-\-cn eye can now develop 

 normal pigmentation; (c) the same transplantation into a f en animal 

 has no effect, because there the v^r substance cannot be trans- 

 formed into cn-\- substance. After Ephrussi and Chevais. 



