An x-rayed apricot [male] crossed to 4 double-yellow attached-X [female][female] produced yellow [female] offspring and 304 phenotypically white [male][male]. Analysis showed the white to be due to the interaction of apricot and a mutation identical or allelomorphic to ruby. This had occurred in the early development of the apricot male in that part of the chromatin destined to enter the germ cells and after the separation from it of the chromatids which were to form the optic anlage. The somatic tissue of the fly was apricot, the germinal tissue apricot-ruby (phenotypically white).