A cross between yellow white males and abnormal abdomen females gave (February 1914) regular results in 10 F_2 cultures, but three cultures gave 2 [female] : 1 [male] sex-ratios (Morgan, 1914_b_, p. 92). The yellow white class, which was a non-cross-over class in these 10 cultures, had disappeared in the 3 cultures. Subsequent work gave the data summarized in table 57. At the time when the results of table 57 were obtained it did not seem possible that two different lethals could be present in the space of about 1 unit between yellow and white, and this lethal was thought to be a reappearance of lethal 1 (Morgan, 1912_b_, p. 92). Since then a large number of lethals have arisen, one of them less than 0.1 unit from yellow, and at least one other mutation has taken place between yellow and white, so that the supposition is now rather that the lethal in question was not lethal 1. Indeed, the linkage data show that this lethal, which may be called lethal 1_b_, lies extraordinarily close to white, for the distance from yellow was 0.8 unit and of white from yellow on the basis of the same data 0.8. There was also a total absence of cross-overs between lethal 1_b_ and white in the total of 846 flies which could have shown such crossing-over. On the basis of this linkage data alone we should be obliged to locate lethal 1_b_ at the point at which white itself is situated, namely, 1.1, but on _a priori_ grounds it seems improbable that a lethal mutation has occurred at the same locus as the factor for white eye-color. Farther evidence against this supposition is that females that have one X chromosome with both yellow and white and the other X chromosome with yellow, lethal, and white are exactly like regular stock yellow white flies. The lethal must have appeared in a chromosome which was already carrying white and yet did not affect the character of the white. We prefer, therefore, to locate lethal 1_b_ at 1.1-. TABLE 57.--_Summary of all linkage data upon lethal 1b, from Morgan, 1914b_. +-------------------------+---------+--------+---------------+ | Gens. | Total. | Cross- | Cross-over | | | | overs. | values. | +-------------------------+---------+--------+---------------+ | Yellow lethal 1_b_ | 744 | 6 | 0.81 | | Yellow white | 2,787 | 23 | 0.82 | | Lethal 1_b_ white | 846 | 0 | 0.0 | +-------------------------+---------+--------+---------------+ FACET.