156 OTTO L. MOHR 



Bridges in Drosophila gynandromorphs (1919). In some 20 cases 

 these authors found through dissection that in flies whose epidermal 

 parts were sex-mosaics, the gonads were the same, i. e., both gonads 

 were ovaries or both were testes. Even in bilateral types the two 

 gonads were alike. This is taken to mean that both gonads in 

 Drosophilu are derived from one and the same common epidermal- 

 germinal nucleus, while with regard to the male here studied the 

 most natural explanation seems to be that one testis is derived from 

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

 from the other. It is, of course, thinkable that one of the two mem- 

 bers belonging to the two-cell stage is the common epidermal-germinal 

 cell mentioned. And if the singed mutation occurred in one of the 

 two daughter X's of this cell, we could also account for the result 

 reached by the genetic test of the mosaic. But this assumption would 

 mean that of the four members of the four-cell stage one was used 

 for the formation of the singed^ part of the individual, including one 

 of the testes, while the three others were used for the formation of 

 the rest of the. mosaic, including the other testis. If this were the 

 case we would, however, in view of the experiences from gynandro- 

 morphs, expect the non-singed^ part of the mosaic to be considerably 

 larger than the singed part, and here we are dealing with a roughly 

 bilateral individual, — though attention may in this connection be 

 called to the fact that the wild-type character of practically the entire 

 head represents a shifting of the bilateral symmetry in favor of the 

 non-singed^ part. 



It will be noticed that there is the possibility that non-virginity 

 of one of the bl pr cii females, by aid of which the Sk cii character 

 of the mosaic was controlled, may account for the result here dis- 

 cussed. These females were derived from a culture in which half of the 

 individuals were bl pr cii and the other half .SA- eu. If one of the 

 females used were fertilized beforehand, she would, of course, give 

 only non-singed^ grandsons, and there was not means of distinguishing 

 her grandsons from those of the singed^ mosaic. 



Full attention was paid to this source of error when the test of 

 the mosaic was carried out, and in order to be absolutely certain 

 three additional females from the homozygous w" v f stock were, as 

 mentioned above, given to the mosaic on the second day. By this 

 double mating method any mistake due to non-virginity of the fema- 

 les would be absolutely excluded, since these females would, if not 

 virgin, give w' v f offspring and, if fertilized by the mosaic, give 



