CROSSINGOVER IN DROSOPHILA 



185 



is necessary to produce the usual effect on the percentage of 

 crossingover. Yet when the high point is reached it is main- 

 tained for approximately the same length of time as the exposure, 

 and not for two days less. The same two day period must elapse, 

 therefore, before the effect of the return to room temperature is 

 registered by a change in the amount of crossingover. This 

 means that what may be termed an 'incubation period' of slightly 

 less than two days' consecutive exposure to the new temperature 

 is necessary before the effect on the developing eggs is produced. 



Percentage of 

 crossing over 



| 1 j 



L 



TO 12 14 16 18 



Days after mating 



Fig. 9. Curves of percentage of crossingover for the black-purple region 



from table 16. = control-hatched and mated at 22°; = hatched at 



22° exposed to 31.5° from the second to the fourth day; = hatched 



at 22° exposed to 31.5° from the second to the sixth day. 



Reference to table 16 shows that the average number of flies 

 hatched during such a period is over a hundred, and perhaps 

 one-third more eggs are laid but do not hatch. It is certain, then, 

 that at least one hundred eggs are normally laid during the 'in- 

 cubation period,' and therefore that in any calculation such as 

 that given above we must subtract the total number of eggs which 

 would be unaffected because of the existence of this 'incubation 

 period.' The corrected value is, then, 125 to 175 eggs. Eggs 

 in such a stage in the ovary that 125 to 175 eggs will be laid 

 before them are without much question in the stage at which a 

 change in the percentage of crossingover due to heat is brought 

 about. 



