The recessive sex-linked mutant yellow body and wing-color appeared in January 1911 (Morgan, 1911_c_ and 1912_a_). Its first appearance was in black stock; hence the fly was a double recessive, then called brown. Later the same mutation has appeared independently from gray stock. Yellow was found to be at the end of the X chromosome, and this end was arbitrarily chosen as the zero or the "left end," while the other gens are spoken of as lying at various distances to the right of yellow. Recently a lethal gen has been located less than one-tenth of a unit (-0.04) to the left of yellow, but yellow is still retained as the zero-point. The viability of yellow is fairly good and the character can be separated from gray with great facility, and in consequence yellow has been used extensively, although at present it is being used less than formerly, since eosin lies only 1.1 units distant from yellow and is generally preferred. ABNORMAL ABDOMEN. (Plate I. figure 4.) The dominant sex-linked character abnormal abdomen appeared in July 1911 (Morgan, 1911_d_). It was soon found that the realization of the abnormal condition depended greatly upon the nature of the environment (Morgan, 1912). Recently a very extensive study of this character has been published (Morgan, 1915). As this case has been reviewed in the introduction, there is little further to be said here. {28} Because of the change that takes place as the culture grows older (the abnormal changing to normal), this character is not of much value in linkage work. The location of the factor in the X chromosome at 2.4 has been made out from the data given by Morgan (1915_b_). These data, which in general include only the abnormal classes, are summarized in table 1. TABLE 1.--_Linkage data, from Morgan, 1915b._ +------------------+-----------+-----------+------------+ | Gens. | Total. | Cross- | Cross-over | | | | overs. | values. | +------------------+-----------+-----------+------------+ | Yellow white | 28,018 | 334 | 1.2 | | Yellow abnormal | 15,314 | 299 | 2.0 | | White abnormal | 16,300 | 277 | 1.7 | +------------------+-----------+-----------+------------+ EOSIN. (Plate II, figures 7 and 8.)