190 



HAROLD H. PLOUGH 



hypothesis. It has been shown positively that crossingover 

 does not occur in the early oogonial divisions, and there is good 

 reason to believe that the percentage of crossingover is affected 

 by temperature at one point in the growth period of the egg only. 

 After this point, only the two polar body divisions take place, 

 and at each of these divisions only one nucleus remains in the 

 egg. The hypothesis cannot therefore explain crossingover in 

 the second chromosome of Drosophila, and must be discarded 

 as a serious attempt to account for the observed phenomena 

 of linkage. 



VII. FAILURE OF TEMPERATURE TO AFFECT CROSSINGOVER IN 

 FIRST AND THIRD CHROMOSOME 



In view of the very positive way in which temperature affects 

 the percentage of crossingover in the second chromosome, it 

 was natural to expect that a similar effect would be found to 

 result from a similar treatment of females which were hetero- 

 zygous for factors in the first and third chromosomes. Strangely 

 enough, the tests so far made show that this is not the case. Using 

 the sex-linked characters, vermilion eyes, sable body color gar- 



TABLE 18* 



HATCHED AT 



Control 22° 



Heat, 32° 



Second broods after 10 



days at 22° 



17.5° 



Second broods after 10 



days at 22° 



per 

 cent 



12.0 



8.8 



11.1 

 12.9 



8.4 



vg 

 sf 



Control 22°. 

 Heat, 31.0°. 



11.1 



10.1 



To save space the non-crossover class is omitted from this table. 



