196 HAROLD H. PLOUGH 
that the masking effect of increased double crossing over is not 
the only reason why certain regions remain unchanged after 
exposure to high temperature. Table 2 shows that in chromo- 
some III the Dichete spineless region (16.4 units) shows an 
increase of more than 100 per cent, while sepia Dichete (11.4 
units) is increased only about 50 per cent, and spineless sooty 
(13.3 units) is practically unchanged. It is apparent that there 
are other factors than the mere length of the region responsible 
for the difference in reaction to high temperature. Not only are 
different chromosomes unlike, but within each chromosome cer- 
tain regions show distinct differences in behavior from certain 
other regions. This fact has been apparent to several Drosophila 
workers in connection with the investigation of the coincidence 
values, and has been the subject of a special study by Bridges, 
the results of which are not yet pubhshed. It is of some interest 
to compare the differences in reaction to temperature with the 
coincidence results. 
Figure 3 shows also the coincidence of double crossing over 
for each pair of adjacent regions as a percentage value outside 
a bracket enclosing the two regions for which it has been calcu- 
lated. The significance of the value for coincidence has been 
discussed in detail by Bridges ('15), Muller ('16), Weinstein 
('18), and Bridges and Morgan ('19). It represents the per- 
centage of the expected number of double crossovers actually 
observed. The size of the coincidence value has been shown 
by a number of observers to be in proportion, up to a certain 
point at least, to the distance apart of the opposite boundaries 
of the regions tested (i.e., to the lack of interference). For 
instance, a glance at the ten-day counts in table 1 shows that 
the coincidence value is high when the scute-echinus and vermil- 
ion-garnet regions (138 per cent) or the scute-echinus and garnet- 
forked regions (86 per cent) are figured, but low when the regions 
are closer together. 
A comparison of the coincidence values given for approximately 
equal lengths of chromosome shows that they do not correspond. 
Double crossing over per unit of distance is apparently much 
