A SOMATIC MITATION IN 1)IU)S01'I1II,A MliLANOfiASTEH 159 



character change inciifioned was due to a sex-linked recessive, which 

 had the same locus as the old sex-linked recessive singed (at 2ü,iij. 

 Somatically the two mutants look entirely alike. 



4. Females homozygous lor the old singed gene are absolutely 

 sterile and lay defective eggs. Homozygous singed' females were 

 found to be of normal fertility, and their eggs were normal. Thus 

 the two genes are not identical, but allelomorphic. The singed-singed^ 

 female compound is also of normal fertility and lays normal eggs. 



5. The fact that we are dealing with a new, previously un- 

 known, allelomorph of singed proves conclusively that the character 

 change found in the mutant part of the mosaic can not be due to 

 contamination. This w^as also demonstrated by aid of the Il-chro- 

 mosome mutant genes present in the mosaic. 



6. It is conclusively demonstrated that the event w liich produced 

 the singed^ mutation occurred after the fertilization of the egg, in one 

 of the very early cleavage nuclei, probably in one of the daughter 

 X's of the dividing egg, or shortly afterw^ards, in one of the two- 

 cell stage nuclei. 



7. When the daughters of the mosaic w-ere outcrossed about 

 half of them transmitted the singed^ mutant gene to half of their sons. 

 The other gave only wild-type sons. Thus, in the mosaic one testis 

 was derived from a cell which had received the singed" Ä', while the 

 other, or part of it, belonged to the wild-type portion of the individual. 

 This seems to indicate that one testis was derived from one of the 

 two daughter cells of the dividing egg, and the other testis from the 

 other daughter cell, a conclusion which is apparently not in accor- 

 dance v'ilh the results derived from the studj^ of gynandromorphs. 

 This point is discussed. 



8. Attention is called to the bearing of cases like the one here 

 described on the mutation conception in general. 



LITERATURE CITED. 



Breitenbechkr, J. K. 1922. Somatic mulalions and elytral mosaics of Bnichus. 



Biol. Bull. Vol. XLIII: 10—22. 

 Bridges, C. B. 1919. The developmental stages at which mutations occur in 



the germ tract. Proc. Exp. Biol, and Med. Vol. XVII: 1 — 2. 

 — 1921. Tripolid intersexes in Drosophila melanogoster. Science, N. S., Vol. 



LIV, No. 1394: 252—254. 

 Emerson, R. -\. 1922. The nature of bud variations as indicated by their 



mode of inheritance. The Amer. A'«/., Vol. LVL No. 642: 64 — 79. 



