CROSSING OVER AND CHROMOSOMES 97 



temperature. Later the crossing over values for inter- 

 mediate points was also obtained, and from these data 

 the curve shown in Fig. 56 was made. 



At a low temperature (about 10° C.) crossing over is 

 increased as compared with a somewhat higher tempera- 

 ture (18-27° C). Room temperature (22° C.) lies in that 

 part of the curve where there is the least amount of cross- 

 ing over. The amount then rises suddenly until about 29° 

 and remains high till 31° C. is reached. The apparent 

 faU after this temperature, as shown by the curve, may 

 not be significant. The flies fail to lay eggs or may die 

 at about this point. 



In the foregoing experiment the eggs, larvaB and adult 

 flies had been kept continuously at the same temperatures. 

 If, however, the heterozygous virgin females reared at 

 high temperature are back-crossed to the triple recessive 

 males, and kept afterwards at normal temperature (22° 

 C.) it is found that only the first ten-day output of such 

 females shows the high crossing over values. The value 

 drops during the following ten days. If a correction 

 is made for a change in the crossing-over value due 

 to age — since age, as Bridges has shown, causes a lower- 

 ing in the value — still the effect of the early period is 

 found to have begun to disappear after ten days, and 

 soon completely disappears. 



In still another way the influence of temperature may 

 be shown. Heterozygous females that had lived at nor- 

 mal temperatures are mated to triple recessive males, and 

 then exposed for the first seven days to 31.5° C. At first 

 the normal crossover values are found, as seen on com- 

 paring Fig. 40 mth Fig. 39 which is the control. The 

 latter drops slightly from the second to the eleventh day. 

 About the eighth day the heat effects begin to show (Fig. 

 40), and there is a sudden and considerable rise in the 

 curve, that lasts for ten days, when it drops back to normal, 

 corresponding with the removal of the flies from the high 

 to normal temperature, i.e., after the seven-day exposure. 

 7 



