EFFECT OF TEMPERATURE ON CROSSING OVER 199 
region is near the middle of the chromosome, "with the spindle 
fiber attachment, and that this middle region is the last part to 
undergo synapsis." Bridges has subsequently applied this same 
idea to chromosome III and decided its middle point is close to 
the locus for Dichete. In the latter case the conclusion as to 
the midpoint of the chromosome has been definitely confirmed 
with the finding by Strong ('20) of the locus for roughoid at 
24.9 units beyond sepia. If, as Bridges suggests, crossing over 
is less in this middle region because synapsis fails or is slight, 
the decreased freedom of crossing over might be consistently 
explained. On the other hand, it should be definitely borne in 
mind that such behavior is an observable phenomenon, which 
is susceptible of cytological demonstration. The demonstration 
that the process of crossing over is accomplished by a simple 
twisting separation, and reunion of chromosome strands is still 
incomplete, and we have no cytological data which indicate 
that in Drosophila the middle region is the last part to undergo 
synapsis. At the early stage in the growth period of the egg 
at which crossing over apparently takes place it seems altogether 
unlikely that the spindle fiber suggested by Bridges is present 
at all. Until we know more of the actual cytological features 
of the crossing-over process and of the spindle fiber attachments 
in Drosophila, such suggestions must be regarded as highly 
speculative. 
AGE AND TEMPERATURE EFFECTS COMPARED 
It has been demonstrated above that in general both age and 
temperature affect the amount of crossing over in the same 
chromosome regions — those probably in which there is a mini- 
mum of crossing over. It is to be expected, therefore, that the 
freedom of crossing over is modified by both agents. It is of 
some interest to note that Bridges and Morgan (p. 199) and also 
Bridges ('19) in identical language conclude that the age variation 
is probably due "to a lengthening of the average length of the 
section of chromosome between simultaneous crossovers," while 
temperature causes an increase in the freedom of crossing over 
with no difference in the length of loop. The clearest evidence 
