188 HAROLD H. PLOUGH 
with excellent viability. The most valuable mutants of each 
chromosome have been assembled into multiple stocks, the use 
of which has made it possible to determine the effect of environ- 
mental changes on linkage relations over approximately the 
whole known lengths of each of these chromosomes in a single 
experiment. At the same time in these multiples the distances 
between the different genes are generally not sufficiently great 
to cause complications due to unobserved double crossing over. 
My present more accurate data establish the truth of the earlier 
observation that crossing over in chromosome I is not influ- 
enced by temperature, but show that- there is a section of chromo- 
some III in which crossing over is increased in the same way as 
in chromosome II. 
EXPERIMENTAL 
The mutant stocks used for the tests were the sex (or first) 
chromosome multiple stock, scute-echinus-cut-vermihon-garnet- 
forked, and the third chromosome multiple stock, sepia-spineless- 
sooty-rough. Scute shows an absence of scutellar bristles, and 
echinus, a roughened condition of the facets of the eyes. The 
other mutations have been described. Dichete, a dominant 
character, was introduced in a small number of preliminary 
third chromosome tests based on ten-day brood counts. A 
glance at the chromosome maps in figure 3 will show that the 
genes used cover both chromosomes at fairly even intervals 
throughout a large portion of their known lengths. 
The method of making the tests was essentially the same as 
that used in my earlier work with the second chromosome. 
Virgin sister females of the normal wild stock were mated to males 
of the multiple mutant stock to be tested, and allowed to lay 
in one set of bottles for about three days. This first set of bot- 
tles was kept at the control temperature. The Pi pairs were 
next transferred to another set of bottles which was kept con- 
tinuously at the high temperature. Virgin female offspring 
from each set were then isolated and back crossed to males of 
the original mutant Stock used. These back crossed pairs were 
placed in quarter-pint milk bottles containing banana agar, and 
