CROSSINGOVER IN DROSOPHILA 



171 



reference to the original records to set the count for approximately 

 the first half of the brood against the second half for comparison. 

 A few such fractionations taken at random gave the following 

 results : 



TABLE 12 



S' 



Heat 31.5°, bred at 22° 



141, First count 



141, First one-half 



141, Second one-half 



141, Second brood after 10 days 

 22° 



145, First count 



145, First one-half 



145, Second one-half 



145, Second brood after 10 days 

 at 22° 



Control 22°, bred at 22° 



150, First count 



150, First one-half 



150, Second one-half 



150, Second brood after 10 days 

 at 22° 



per cent 



30.8 

 32.0 

 29.1 



15.6 



30.7 

 35.5 



20.8 



16.4 



18.7 

 20.0 

 14.4 



13.7 



In both of the bottles seeded by females which have been 

 exposed to heat during their development, we find that the 

 offspring hatched during approximately the first half of the 

 period counted show a higher percentage of crossingover than 

 those hatched later. The latter approach the value given by 

 the second broods, which has already been shown to be practi- 

 cally the same as the second brood control value. Fractionation 

 of a control culture shows a similar progressive decrease, though 

 it is less striking because the difference between the first and 

 second brood values is much less. Fractionation of the first 

 ten day broods brings out then that the heat effect is more 



