178 



HAROLD H. PLOUGH 



question that this value varies between a maximum and a mini- 

 mum separated by six or seven units, and that this variation has 

 a perfectly definite rhythm. The control value always starts 



Percentage of 

 crossing over 



24 

 22 



Purple-curved 



Black-purple 



Days after mating 



8 10 12 14 16 18 20 22 24 26 28 30 



Fig. 6. Curves of percentage of crossingover for both regions shown by the 

 offspring of females hatched at 31.5° but mated at 22°. Broken line indicates 

 percentages from three pairs returned to the high temperature on the tenth day 

 (table 13). 



Percentage of 

 crossing over 



20 

 18 



16 

 14 

 12 

 10 

 8 



Days after mating 



~8 10 15! 14 16" 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 



Fig. 7. Curves of percentage of crossingover for the black-purple region 



from table 14. = control-hatched and mated at 22°; = hatched at 



22° — exposed to 31.5° from the third to the eleventh day. ■ = six 



control pairs exposed to 9° from the twenty-first to the twenty-eighth day (no 

 eggs laid during the interval). 



