CROSSINGOVER IN DROSOPHILA 



179 



Percentage of 

 crossing over 



32 



30 



Days after mating 



ugya quel iiipu m^ 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 



Fig. 8. Curves of percentage of crossingover for the purple-curved region 

 from table 14. (Legend as in figure 7.) 



rather high, and drops gradually to a low point which is reached 

 between the 7th and the 10th day. It remains at close to this 

 point until about the sixteenth day, when a rise begins, which 

 brings it up again to the maximum after about twenty-two to 

 twenty-four days. A second drop brings the value down again 

 before the thirtieth day. In the longer curves there is even a 

 suggestion of a second rise, but after about the thirtieth day the 

 numbers are too small to be of great significance. By cutting 

 such a curve into ten day periods a series of percentages can be 

 secured which corresponds to those recorded by Bridges, and con- 

 firmed in this paper. The first ten day brood catches the initial 

 high point and the first drop; the second brood, the minimum and 

 the beginning of the first rise, while the third ten day period 

 includes the second high point and the second drop. The first 

 brood would give the highest value, the third the next highest 

 (if the second maximum is not quite as high as the initial one) 

 and the second the lowest. This 'curve of age' must be inti- 

 mately connected with certain physiological processes in the 

 body of the female which have some 'environmental' effect on 

 the developing eggs. The first drop in the curve might be ex- 

 plained by the gradual change through which the body goes 

 during the metamorphosis, when the eggs which register the 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 24, NO. 2 



