NEW DATA. 



55 



The data give 43 per cent of crossing-over, which places fused far to 

 the right or to the left of eosin. The latter position is improbable, 

 since eosin already lies very near the extreme left end of the known 

 series. Therefore, since 43 per cent would place the factor nearly at 

 the right end of the series, the next step was to test its relation to a 

 factor like bar that lies at the right end of the chromosome. By mating 

 to bar alone we could only get the linkage to bar without discovering on 



FIG. D. a, normal wing; b and c, fused wings, c shows a typical fused wing. The most striking 

 feature is the closure of the cell between the third and fourth longitudinal veins with the 

 elimination of the cross- vein; the veins at the base of the wing differ from those in the normal 

 shown in a. b shows the normal position in which the fused wings are held. The fusion of 

 the veins in b is unusually complete. 



which side of bar the new factor lies, but by mating to a fly that carries 

 still another sex-linked factor, known to lie to the left of bar, the infor- 

 mation gained should show the relative order of the factors involved. 

 Furthermore, since, by making a back-cross, both males and females 

 give the same kind of data (and need not be separated), the experiment 

 was made in this way. In order to have material for such an experi- 

 ment double mutant stocks of vermilion fused and also of bar fused 

 were made up. 



