TABLE 33.--_P_1 eosin [female][female] x fused [male][male]. F_1 wild-type [female][female] x F_1 eosin [male][male]._ +----------+--------+-----------------+----------------+-------+--------+ | | | Non-cross-over | Cross-over | | | | | | [male][male]. | [male][male]. | Total | Cross- | |Reference.|Females.+--------+--------+--------+-------+ males.| over | | | | Eosin. | Fused. | Eosin | Wild- | | value. | | | | | | fused. | type. | | | +----------+--------+--------+--------+--------+-------+-------+--------+ | 56I | 496 | 131 | 113 | 82 | 104 | 430 | 43 | +----------+--------+--------+--------+--------+-------+-------+--------+ {54} 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 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 information 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 experiment double mutant stocks of vermilion fused and also of bar fused were made up. [Illustration: 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.] {55} LINKAGE OF VERMILION, BAR, AND FUSED. Males from the stock of (red) bar fused were mated to vermilion (not-bar, not-fused) females, and produced bar females and vermilion males. The bar F_1 daughters were back-crossed to vermilion fused males and produced the classes of offspring shown in table 34. TABLE 34.--P_1 _vermilion_ [female] [female] x _bar fused_ [male] [male]. _B. C. F_1 bar_ [female] x _vermilion fused_ [male] [male].