A Semi-lethal in Drosophila funebris. 



95 



eyes and other head parts. Organs may entirely fail to arise, or 

 develop abnormally after they do arise. 



In the series of double fish when both individuals or both heads, 

 as the case may be, are of equal size they are both normal, but 

 whenever one component is larger than the other, the larger one is 

 almost invariably normal and the smaller is invariably defective. 

 This is not only true in the present series of specimens but also in 

 all illustrations and descriptions of double monsters which I have 

 been able to collect from the literature. 



These embryos furnish material for an analysis of the causes 

 of many common structural defects about which there has been 

 considerable discussion, a consideration of this phase of the subject 

 will be given in the complete review of the experiments. 



55 (1430) 



A semi-lethal in Drosophila funebris that causes an excess of 



males. 



By O. L. Mohr and A. H. Sturtevant (by invitation). 



[From the Zoological Laboratory of Columbia University, New 



York City.} 



In the course of the genetic work on Drosophila melanogaster 

 cases have been found rather frequently in which the sex ratios 

 showed marked deviations from the usual approximate equality. 

 Those cases in which there is a deficiency of males have been 

 the most frequent, and the explanation of many of these has been 

 worked out by Rawls, Morgan, Bridges, Stark, and others. They 

 are now known to be due to sex-linked lethal genes. Less fre- 

 quently cases have been noted in which there was a deficiency of 

 females (see Quackenbush, Science vol. 32). In none of these 

 has the explanation hitherto been discovered. 



From a culture of Drosophila funebris we obtained one female 

 and 87 males. This female, mated to a few brothers, produced 

 60 females to 103 males. Descendants of this mating have been 

 inbred for many generations, and have given sex ratios ranging 

 from o 9 : 76 o 71 up to approximate equality. There is no obvious 



