192 HAROLD H. PLOUGH 



1. Wet and dry food, starvation, increased fermentation 

 of the food, and probably solutions of F 2 C1 3 are ineffective in 

 causing a change in the percentage of crossingover among the 

 offspring of a female to which these environmental influences 

 are applied. 



2. A temperature of 31°C. and of 13°C. applied to female 

 flies while they are undergoing development from egg to adult 

 causes a decided increase in the percentage of crossingover among 

 the offspring of the first brood, but not of the second brood 



3. Tests of offspring of females treated as in No. 2 above 

 at an ascending series of temperatures gives a curve which rises 

 from 9°C. to a first maximum of 13°C, drops through 17.5°C. to 

 a minimum extending from 22°C. to 27°C, rises again through 

 29°C. to a still higher maximum at 31°C, after which a slight 

 fall is again noted. This indicates some sort of a change in the 

 physical state in the structural makeup of the nuclear mechanism. 



4. More detailed analysis shows that the high percentage of 

 crossingover can be induced among the offspring by at least two 

 consecutive days' exposure of the females to high temperature 

 from their late larval period on. If the females are exposed 

 before hatching the increase is noted among their first offspring, 

 but if the exposure comes after the adult stage is reached the 

 high percentage does not appear until between 225 and 275 

 eggs have been laid. The percentage of crossingover remains 

 at the high point for approximately the same number of days 

 that the female parent was exposed to the high temperature, anjd 

 then drops to the control value. 



5. The facts given in No. 4 are taken to indicate that the 

 high temperature influences crossingover at one point in the 

 oogenesis only. Since nearly two consecutive days' output of 

 eggs is to be subtracted from the total given above, due to the 

 existence of an 'incubation period,' we may consider a stage in 

 oogenesis which is so situated that between 125 and 175 eggs in 

 the ovary are more advanced than this stage, represents approxi- 

 mately the point at which crossingover takes place. 



6. Actual examination shows that there are on an average 

 140 oocytes present in a female which is newly hatched. There 

 is, therefore, good ground for the belief that crossingover occurs 



