142 



PHYSICAL BASIS OF HEREDITY 



between 10° C. and 20° C. It then rises rapidly, reaching 

 a higher maximum than the first at about 28° C, after 

 which it decreases until rigor sets in at 38° C. 



The results of crossing over between purple and curved 

 gave similar results, but the ^* distance" here is so great 

 that double crossing over complicates the results ; there- 

 fore they need not, for the present, be analyzed further. 

 Attempts to change the crossing over value by starvation, 

 moisture, increase in fermentation of the food, iron salts, 

 etc., gave no results that seemed significant. On the other 



% 



cro%: 



ntgreta r 



it 31 32 



Fig. 56. — Curve showing influence of crossing over at different temperatures. (After Plough.) 



hand. Bridges had already noted that a decrease in the 

 amount of crossing over is found in second broods as 

 compared with first broods — ten-day periods. AYliat 

 change in the environment is behind this ''age'' dif- 

 ference is not clear, but since most of the eggs pass 

 through this early prematuration stage in the lar^^se 

 and some of them may reach the maturation stage 

 in the pupa, it is possible that prevailing conditions in 

 one or the other of these physiological states may be 

 responsible for the difference between these states and 

 those that prevail after the fly has hatched. 



