The position of lethal _sb_ was accurately found by continuing the determinations with a white lethal cross-over. A white female was found which had only one of the two lethals and the linkage of this lethal with eosin and miniature was found as follows: A female carrying white and lethal in one chromosome and no mutant factor in the homologous chromosome was bred to an eosin miniature male. The white eosin daughters carried lethal, and their sons show the amount of crossing-over between white and lethal (15.6), between lethal and miniature (19.9), and between white and miniature (32.9). The data on which these calculations are based are given in table 48. TABLE 48.--_Data on the linkage of white, lethal sb, and miniature, from Stark, 1915_. +-----------+------------+------------+--------------+ | w^e m | w^e l_{sb} | w^e | w^e l_{sb} m | | --------- | ---+------ | -------+-- | ---+-----+-- | | w l_{sb} | w m | w l_{sb} m | w | | | | | | +-----------+------------+------------+--------------+ | Eosin | White | Eosin. | White. ~ | miniature | miniature. | | ~ | | | | | +-----------+------------+------------+--------------+ | 2,421 | 524 | 685 | 48 | +-----------+------------+------------+--------------+ +--------+--------------------------------+ | | | | | Cross-over values. | | | | | | | | Total. +----------+----------+----------+ ~ |White |Lethal |White | ~ |lethal |_sb_ |miniature.| | |_sb_. |miniature.| | +--------+----------+----------+----------+ | 3,678 | 15.6 | 19.9 | 32.9 | +--------+----------+----------+----------+ {65} The locus of this lethal is at 16.7; the locus of lethal _sa_ was found to be at 23.7, so that the lethal at 16.7 is evidently the second lethal or lethal _sb_ whose advent gave rise to the high sex-ratio. This interpretation is in accord with the curve which Miss Stark published, for although the mode which corresponds to lethal _sa_ is weak, the mode at 15-16 is well marked. The two other lethals, lethals _sc_ and _sd_, which came up in the course of these experiments by Miss Stark, are treated in other sections of this paper. BAR. (Plate II, figures 12 and 13.)