A SOMATIC MUTATION IN DHOSOPIIILA MELANOGASTER 155 



allelomorphic. This fundamental difference between the old singed 

 and the new singed females with regard to fertility, probably means 

 that tiie old singed gene in homozygous females, in addition to the 

 external charaefi'r changes, also produces internal alterations which 

 prevent the development of normal eggs, alterations which are not 

 produced by the new allelomorph. 



Finally, the fact that the new mutant gene is not identical with 

 the old singed gene but an allelomorph thereof proves abundantly 

 clear, that the origin of the bristle alteration in the mosaic here descri- 

 bed can not be explained in any way as due to contamination. We are 

 dealing with an entirely new, previously unknown mutation. 



Two independent mutations in the singed locus, which both 

 caused sterility of the homozygous females, have earlier been descri- 

 bed by the author. The somatic mutation here recorded, being num- 

 ber three in the series, was accordingly called »singed^ {sn^j. 



VI. DISCUSSION. 



'The tests presented above prove conclusively that the character 

 change found in the exceptional male was due to a recessive sex-linked 

 mutation. This mutation was not identical with but allelomorphic 

 to the old singed mutation. 



As to the stage in the life cycle at which the mutation arose, 

 we know with certainty that the event which produced the change in 

 the singed locus must have taken place after the fertilization of the 

 egg. The zygote received an unmutated maternal Ä'-chromosome. 

 This is demonstrated by the fact that about half of the individual 

 had normal bristles and hairs. Since a large region of the body was 

 singed^, it seems clear that the mutation must have occurred in one 

 of the very early cleavage nuclei. The roughly bilateral distribution 

 ♦)f the singed" and the wild-type regions favors the view that the 

 mutation arose in one of the daughter X's of the dividing egg nucleus, 

 or, shortly afterwards, in one of the two-cell stage nuclei. 



The tests indicated further that one of the testes of the mosaic 

 was built up of cells which had received the singed^ A', while the 

 other had the normal, unmutated A'-chromosome. This seems also to 

 be in good accordance with the fact that exactly half of the abdominal 

 epidermis, including half of the external genitalia, was singed^ and 

 the other half normal. 



The latter result differs from the one reached bv Morgan and 



