194 HAROLD H. PLOUGH 
sooty rough shows a rise of nearly six units in the four-to-seven- 
day period, but since no difference appears either before or after 
this time, it is probable that this has no significance. The 
data indicate, therefore, that the percentage of crossing over is 
increased by exposure to high temperature at one end of chromo- 
some III, but not throughout the remainder of its length. 
It is interesting to note that the control line for the sepia 
spineless region shows the age variation observed in the second 
chromosome. The value gradually drops to the tenth day and 
then rises. The rise apparently takes place somewhat earlier 
than in chromosome II. The other two regions show no signifi- 
cant difference as the female grows older. 
The results of the tests may be summarized as follows: a) the 
sex chromosome shows no significant increase in the percentage 
of crossing over as a result of the exposure of the developing 
eggs to high temperature; b) the third chromosome shows an 
increase in crossing over in the sepia spineless region, but nowhere 
else; c) a variation in crossing over with the age of the female 
occurs in those regions which show a reaction to temperature only. 
REACTION TO TEMPERATURE AND HIGH COINCIDENCE 
In figure 3 I have drawn to the same scale comparative maps 
of the principal chromosome regions whose reactions to high 
temperature have been tested. The percentages of crossing 
over in chromosome I have been calculated from the ten-day 
brood counts summarized at the end of table 1, and those for 
chromosome III from table 2. The map of chromosome II is 
taken (for the points indicated) directly from the very accurate 
one given by Bridges and Morgan (p. 302). The regions for 
which a rise in the percentage of crossing over as a result of 
exposure to a temperature of 31.5°C. has been recorded — either 
in this or my former paper — are indicated by diagonal Unes, 
while those which are not changed are solid black. As noted 
previously, it may develop that one or both of the long regions 
at either end of chromosome II will show a reaction to temper- 
ature if they can be broken into short blocks. A rise in total 
crossing over may be obscured by a compensating rise in double 
