182 



HAROLD H. PLOUGH 



tion, namely, that the high or low temperature has an effect 

 on the chromosomal mechanism at one point in the development 

 of the eggs only, and that eggs which have already passed this 

 particular point in their development when the new temperature 

 is applied, or which do not reach that point during the period 

 of exposure, register no higher percentage of crossingover than 

 the controls. 



VI. LOCALIZATION OF PROBABLE POINT OF CROSSINGOVER IN 



OVARIES 



Genetic data 



From the data given in tables 13 and 14 it is possible to locate 

 the point where- this effect is produced with fair numerical 

 exactness. One has only to add the sums of the average tube 

 counts for the periods from the time that heat was applied to the 

 time when the first significant rise in the curve takes place. 

 The same total ought to be given by adding the sums of the 

 periods between the time when the exposure is discontinued to 

 the time when the drop to the control value occurs. Such an 

 operation gives the following results: 



TABLE 15 



Figure 5, table 13 



Figure 6, table 13: 



first drop 



second rise 



Figure 7 and 8, table 14 



NUMBER OF FLIES 



HATCHED BEFORE 



EFFECT 18 



SHOWN 



284 



187 

 197 



NUMBER OF FLIES 

 HATCHED BEFORE 

 EFFECT DIS- 

 APPEARS 



121 

 240 

 224 



A glance at the tables shows that some of these figures must 

 be smoothed considerably. In figure 5 only one count was 

 taken for the first seven days. It seems certain that if this 

 period had been broken up the rise would have begun sooner, 

 and the total of 284 would have been much less. The same 

 condition obtained to a less degree with the second total — 240 

 — which is probably not much too high. The very low value of 



