There are three loci involved in this cross, namely, cherry, lemon, and vermilion. Of these loci two were known, cherry and vermilion. The data are consistent with the assumption that the lemon locus is between cherry and vermilion, for the double cross-over classes (the smallest classes) are cherry lemon vermilion and wild type. The number of single cross-overs between cherry and lemon and between lemon and vermilion are also consistent with this assumption. Since lemon flies fail to emerge successfully, depending in part upon the condition of the bottle, the classes involving lemon are worthless in calculating crossing-over and are here ignored. In other words, lemon may be treated as though it did not appear at all, _i. e._, as a lethal. The not-lemon classes--cherry, vermilion, cherry vermilion, and wild type--give the following approximate cross-over values for the three loci involved: Cherry lemon, 15; lemon vermilion, 12; cherry vermilion, 27. The locus of lemon, calculated by interpolation, is at about 17.5. LETHAL 2. In September 1912 a certain wild female produced 78 daughters and only 16 sons (Morgan, 1914_b_); 63 of these daughters were tested and 31 of them gave 2 females to 1 male, while 32 of them gave 1:1 sex-ratios. This shows that the mother of the original high sex-ratio was heterozygous for a recessive sex-linked lethal. In order to determine the position of this lethal, a lethal-bearing female was bred to an eosin (or white) miniature male, and those daughters that were heterozygous for eosin, lethal, and miniature were then back-crossed to {50} eosin miniature males. The daughters that result from such a cross give only the amount of crossing-over between eosin and miniature (as 29.7), but the males give the cross-over values for eosin lethal (9.9), lethal miniature (15.4), and eosin miniature (25.1). The data for this cross are given in table 25. TABLE 25.--_Total data upon linkage of eosin, lethal 2, and miniature, from Morgan, 1914b._ +------------------------------------+ | Females. | +--------+--------------+------------+ | | | | | Total. | Cross-overs. | Cross-over | | | | value. ~ | | | ~ +--------+--------------+------------+ | 15,904 | 4,736 | 29.7 | +--------+--------------+------------+