270 LILIAN V. MORGAN. 



males (XY) would survive, and one class (YY) die (Fig. 2). In 

 subsequent generations the line of double yellow females has re- 

 mained intact ; crosses have been made to many more types of 

 males, always producing males like the father. The double yellow 

 females are now used when it is desired to keep a supply of a cer- 

 tain type of male, for, since the X-chromosomes are inseparable, 

 any egg which receives them produces a female, and only those 

 eggs which receive the father's X can produce males. In such a 

 line, then, there is never any crossing over in the X-chromosomes, 

 and any desired combination of sex-linked characters can be main- 

 tained in the male flies by mating a male having the characters to 

 a double yellow female. This is useful in cases where females of 

 the race are weak or sterile, or where the pure cultures of the 

 desired characters are weak, as frequently happens with a compli- 

 cated combination of characters. The double yellow females are 

 very vigorous and afford a reliable medium upon which to rear a 

 race of feeble males. A possibility of a break in the continuity of 

 a pure culture, which is easily discovered and rectified, will be dis- 

 cussed later. 



The double yellow females probably behave like other flies in 

 regard to the inheritance of characters in the autosomes. The 

 original mosaic was dichete and transmitted the character normally. 

 Crossing over in the second chromosome was tested by mating 

 double yellow females to black purple vestigial lobe males and was 

 found to be normal. Crossing over in the third chromosome was 

 tested by mating double yellow females to pink, spineless, kidney, 

 sooty, rough males, and found to be normal at the left end of the 

 chromosome, but not at the right end ; the discrepancy may, how- 

 ever, be due to the difficulty in classifying sooty in the presence of 

 yellow. 



X-TRIPLOIDS. 



After many hundreds of flies had been examined, there appeared 

 occasionally, in cultures where the males were gray, wild type 

 females of a more or less abnormal appearance; the eyes were 

 somewhat rough and smaller than normal, the wings more or less 

 imperfect and sometimes serrated, the abdomen imperfect, and the 

 flies were weak (Fig. i, &). These are characteristics of females 



