232 CALVIN B. BRIDGES. 



do not preclude the simultaneous use of other mutants. On the 

 other hand, the use of such an extreme mutant character as 

 white-eyes prevents the effective use in the same experiment of 

 all other eye colors, and likewise the use of vestigial-wing hinders 

 the use of all other wing and venation characters. The most 

 valuable mutants are, then, those of slight but definite somatic 

 change, free from disturbance to viability or masking effects on 

 other mutant characters. 



ORIGIN AND DESCRIPTION OF THE WHITE-OCELLI MUTANT. 



One of the best examples of a slight mutant character that 

 fulfills these conditions is that of "white-ocelli," which was found 

 very early in the breeding work with Drosophila (June 21, 1912). 

 The ocelli of wild flies are three small simple eyes in a group on 

 the dorsal posterior part of the head. In color they are of a 

 dilute brownish-red, about that of a coffee infusion. Close 

 examination shows that the color of the ocelli themselves is 

 quite light, about that of weak tea, but that there is a crescent" 

 shaped deposit of dense brownish-red pigment about the median 

 side of the two posterior ocelli and against the posterior side of 

 the anterior ocellus. The apparent color of the ocellus is largely 

 due to this outside pigment seen through the transparent lens-like 

 ocellus, and consequently the color changes in intensity with the 

 angle at which the ocellus is viewed. 



It was observed that the ocelli of white and also of vermilion- 

 eyed flies were without color or pigment deposit. These ocellar 

 changes are only other effects of the white and vermilion genes. 

 That the color of the ocelli could vary independently of that of 

 the eyes became apparent when it was found that about half 

 of the flies of the stock of the mutant black-body color had 

 white ocelli, while the remainder had the normal coffee-colored 

 ocelli. Some of the white-ocelli flies were bred together and 

 gave a pure-breeding stock of white-ocelli flies (June, 1912). 

 About a year after this, it was found (July, 1913) that the stock 

 of the third-chromosome recessive spineless was also pure for 

 the white-ocellar color. The gene for white-ocelli was thought 

 to be in the third chromosome, since, in crosses in which the 

 spineless was used, the white-ocellar character generally reap- 

 peared in association with the spineless, though sometimes not. 



