CROSSINGOVER IN DROSOPHILA 181 



Figures 7 and 8 show the same facts with larger numbers and a 

 value for each two day period. The broken line shows the values 

 for the offspring of females hatched as in figure 5 from the same 

 cultures as the controls. They were mated and kept for three 

 days at 22°C. At this point they were exposed to a temperature 

 of 31.5°C. which was continued for eight days. The curve 

 continues at approximately the same level as the control for 

 eight days, and then rises in two distinct steps. This high point 

 is maintained for about eight days when the curve comes down 

 with considerable abruptness to about the control value, where 

 it remains. 2 At the twenty-first day the control series was 

 divided and six tubes were subjected to a cold test of 9°C. for 

 seven days. During this period no eggs were laid and there- 

 fore no results are recorded. On the thirty-first day, however, a 

 rise was begun and the curve was maintained at a point well 

 above the control for seven days. Then a drop occurred, fol- 

 lowed by a second but smaller rise, after which the control value 

 was again reached. This last rise would seem to mean that 

 when as low a temperature as 9°C. is used a slight heat stimula- 

 tion results by transferring the flies back to normal temperature. 



The facts which these tables and curves bring out are briefly 

 these. Exposure of a female to a temperature of 31.5°C. for a 

 given period results in a decided increase in the percentage of 

 crossingover among her offspring for a period about as long as 

 the exposure. In case she is exposed up to the time of hatching 

 this increase shows among the offspring of the first seven or 

 eight days after mating. If the adults are exposed, the effect 

 begins to be noted at the seventh or eighth day after the adults 

 first began to receive the treatment. The increase is main- 

 tained for the definite period mentioned above only. When the 

 value has again returned to that of the control no further devia- 

 tion is observable, though the rhythm of the 'curve of age' seems 

 to have been disturbed. The facts admit of only one interpreta- 



2 This series was subjected to a second exposure to 31.5°C. from the 21st to 

 the 23d day. A slight rise was recorded in the curve about the 27th day. The 

 failure to show the usual maximum for an exposure of only two days is discussed 

 in a later section. 



