Rudimentary, which appeared in June 1910, was the second sex-linked character in _Drosophila_ (Morgan, 1910_c_). Its viability has always been very poor; in this respect it is one of the very poorest of the sex-linked characters. The early linkage data (Morgan, 1911_a_) derived from mass cultures have all been discarded. By breeding from a single F_1 female in each large culture bottle it has been possible to obtain results which are fairly trustworthy (Morgan, 1912_g_; Morgan and Tice, 1914). These data appear in table 65, which summarizes all the published data. {26} The locus of rudimentary is at 55.1, for a long time the extreme right end of the known chromosome, though recently several mutants have been found to lie somewhat beyond it. [Illustration: Fig. A. _a._ rudimentary wing; _b._ the wild fly for comparison.] The rudimentary males are perfectly fertile, but the rudimentary females rarely produce any offspring at all, and then only a very few. The reason for this is that most of the germ-cells cease their development in the early growth stage of the eggs (Morgan, 1915_a_). MINIATURE. (Plate II. figures 7 and 8.) The recessive sex-linked mutant miniature wings appeared in August 1910 (Morgan, 1911b and 1912a). The viability of miniature is fair, and this stock has been used in linkage experiments more than any {27} other, with the single exception of white. While the wings of miniature usually extend backwards, they are sometimes held out at right angles to the body, and especially in acid bottles the miniature flies easily become stuck to the food or the wings become stringy, so that other wing characters are not easy to distinguish in those flies which are also miniature. At present vermilion, whose locus is at 33, in being used more frequently in linkage work. The locus of miniature at 36.1 is slightly beyond the middle of the chromosome. VERMILION. (Plate II. figure 10.) The recessive sex-linked mutant vermilion eye-color (Morgan, 1911_c_ and 1912_a_) appeared in November 1910, and has appeared at least twice since then (Morgan and Plough, 1915). This is one of the best of the sex-linked characters, on account of its excellent viability, its sharp distinction from normal with very little variability, its value as a double recessive in combination with other sex-linked eye-colors, and because of its location at 33.0, very near to the middle of the known chromosome. YELLOW. (Plate I. figure 5.)