98 



PHYSICAL BASIS OF HEREDITY 



From data of this kind it is possible to locate the stage 

 in the development of the egg when the heat is affecting it. 

 If, for instance, we know how long after subjecting a 



Percentage of 

 crossing over 



2I[ 

 19 



17 

 16 



13 



II 



9 



7 

 6 

 S 



1 



J~T 



■^ Purple-curved 



^-lJ 



- BiacK-purpie 



8 10 \2 14 16 18 20 22 24 26 20 30 32 34 36 



Days 



after mating 



Fig. 39. — Curve showing the influence of temperature on crossing over; control 



(After Plough.) 



Percentage of 

 crossing over 



84 



84 



22 



20 

 18 

 16 

 14 



12 



10 

 8 



6 



Purple-curved 



Black-purpie 



Days after mating 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 

 FiQ. 40. — Curve showing the influence of temperature on crossing over. (After Plough.) 



female to a high temperature, the effect of heat on crossing 

 over begins to be observed in her offspring, and also how 

 many eggs are laid by the female before this influence 

 is manifest, we can tell approximately in what stages heat 



