24 THE ORIGIN OF GYNANDROMORPHS. 



from white-eyed stock. He showed that these flies respond much less 

 actively to hght than do red-eyed flies. In these red-white mosaics 

 the red eye, giving a stronger positively phototropic reaction, turns 

 the fly toward that side. Of course, if the fly turns toward a single 

 source of illumination, such as a window or artificial light, the red 

 eye will soon pass into its own shadow as the fly turns, and the con- 

 dition on the two sides may become balanced, unless the general 

 illumination from the wall of the room, for instance, is still stronger 

 than the influence of the window's light on the white eye. In order 

 to avoid this complication the fly should be kept on a vertical surface 

 held at right angles to the light, when its circus movements are not 

 interfered with by the opacity of its own body. 



Since the male side of the body, including the legs, is generally 

 smaller than the female side, and since the male side is the one that 

 has the white eye, there is a chance that the movements toward the 

 red side are against the stronger action of that side. This complica- 

 tion was, however, not realized in all the cases in which circling occurred, 

 but since in several of them the legs on the right and left sides were the 

 same it is practically certain that the results are largely, if not entirely, 

 due to the difference in stimulus from the two eyes. 



SEX-LIMITED MOSAICS. 



By a sex-limited character (in contradistinction to a sex-linked) 

 we mean a character that is peculiar to one or the other sex, but is 

 not necessarily transmitted by means of a gene in the sex chromosome. 

 Such a character is shown by a stock called white tip, in which the 

 pigment bands are absent from the last segments of the abdomen in 

 the female but not in the male. In this stock a gynandromorph 

 arose (text-fig. 6), male on the left side and female on the right. On 

 the male side the black tip to the abdomen is present, although here, 

 as in the stock itself, it is not as black as in the wild type. On the 

 female side the abdomen has a white end. 



In this case elimination of a sex chromosome produced the gynan- 

 dromorphous condition, and since in this stock the female parts are 

 different from the male, owing to a factor presumably not in the sex 

 chromosome, the right side of the gynandromorph also shows this 

 peculiarity, owing to its femaleness. 



A similar case appeared (No. 2864, Jan. 1915) in a cross between a 

 faint-band female and a star faint-band male. Faint-band is a sex- 

 linked character which appears only in the female. All of the flies 

 of the above cross were pure faint-bands; but while the females were 

 characterized by abdominal bands in which both chitinization and 

 pigmentation were weak and by short, slender, and irregular bristles 

 throughout, the males could not be distinguished from wild males 

 in appearance. The gynandromorph was completely bilateral, the 



