EFFECT OF TEMPERATURE ON CROSSING OVER 189 . 
kept at the control temperature. With the exception of the 
preUminary ten-day brood test, the pairs were changed from 
one set of such bottles to another at the end of successive three- 
day periods throughout the hfe of the females. (The first 
change was made in all cases at the end of the fourth day.) The 
counts of the successive sets of offspring of these pairs furnished 
the data for determining the effect of the high temperature on 
crossing over in the developing eggs of the heterozygous females. 
The control temperature was approximately 24°C. maintained 
in a wooden stock cabinet controlled by an electric heater with 
a thermostat. It varied between 22°C. and 25°C. throughout 
the experiments. The high temperature was 31.5°C. maintained 
in a Freas electric incubator. This varied as much as 1° above 
and below, though the normal variation was about 0.5° either 
way. 
The results given by the tests of the first chromosome regions 
are given in table 1. The bottle counts for each three-day period 
are added and the percentages of crossing over for each of the 
five regions calculated and listed in the columns at the right. 
The successive percentages of crossing over for each of the five 
regions are plotted as curves in figure 1, the dotted line in each 
case being the experimental value and the full line the control. 
The results of the tests of the multiple third chromosome stock 
are given in tables 2 and 3, with the percentages of crossing over 
in the columns headed per cent 1, etc. Table 2 summarizes the 
results of a ten-day brood count made with the sepia-Dichete- 
spineless-sooty-rough stock, and table 3 shows the three-day- 
interval results from the same stock without Dichete. The 
crossover values of table 3 are plotted as curves in figure 2. 
INTERPRETATION OF THE CURVES OF CROSSING OVER 
An examination of the curves in figure 1 demonstrates the 
following facts. First, the full and dotted lines for each of the 
five regions show no significant differences. The work on the 
second chromosome showed that for a region that was sensitive 
to temperature, the dotted line was significantly higher than the 
