198 HAROLD H. PLOUGH 
DISCUSSION 
The fact that high coincidence and sensitiveness to environ- 
mental effects are found in the same chromosomal regions sug- 
gests that certain structural features of the crossing over process 
determine each. Bridges and Morgan (p. 188) suggest that 
the difference in the amount of interference for short regions 
between the first and the second chromosomes may be inter- 
preted in two ways. We may assume that the average length 
of loop between simultaneous crossovers is the same in each, 
which means that a region having a given coincidence value in 
chromosome II is actually the same length as one having the 
same coincidence in chromosome I. On the other hand, we may 
hold that the length of loop between simultaneous crossovers 
is relatively shorter in chromosome II than in chromosome I, 
which means that equal amounts of crossing over then indicate 
equal lengths of chromosome. Either of these alternatives holds 
also for the different sections of chromosome III. According 
to the former interpretation, crossing over takes place relatively 
less freely in the regions ruled with the diagonal lines and they 
are actually much longer than the map length indicates. Ac- 
cording to the latter view, crossing over takes place relatively 
more freely, and the map lengths are accurate. Bridges ('19, 
and from subsequent unpublished data) distinctly favors the 
former interpretation. The effect of high temperature in causing 
an increase in these regions does not give any clear evidence 
for either view, though it would seem to support Bridges' inter- 
pretation. It is hardly possible that temperature does not act 
on the whole chromosome equally. Any observed differences 
between different regions would seem to be due to the fact that 
slight effects are registered in certain regions and not in others. 
It is reasonable to suppose that the regions in which a change 
is observable should be those in which crossing over is less free. 
It is of some interest to consider what structural conditions 
in the chromosomes could result in regions of decreased freedom 
of crossing over. Bridges and Morgan (p. 198) and Bridges 
('19) suggest that the reason for the difference in behavior of the 
black curved region in chromosome II may lie in the fact that this 
