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 



PercentaQe of 



crossing .over 



2lf 



-J—L 



• Purple-curved 



JUl 



• Black-purple 



■SS- Days 



2 * a a lb 112 14 16 IB 20 22 24 26 28 30 32 34 36 g,,^^ ^'gji^^ 



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

 (After Plough.) 



Percentage of 

 Classing over 



e4 



22 



so 



Purple-curved 



BlacK-purpie 



Days after mating 



2 4 6 8 10 12 14 IB IB 20 22 24 26 28 30 32 34 3S 



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 



