An Experimental Study of Somatic Modifications etc. 327 
discovered to be pregnant. The discovery vras made, on the average, 
about five days before the birth of the young, i. e. about two weeks 
after the actual commencement of pregnancy. Thus some indirect 
influence of the external temperature conditions upon the developing 
fetus is at least conceivable. Such a possibility will be considered 
later. At present I shall merely point out that at the time of the 
pairing (May 2) the differences in temperature between the two rooms 
had diminished greatly (Fig. 1) and that they continued to diminish 
rapidly. The average difference from May 2 to May 29 (when the 
last of the parent mice were brought into a common room) was only 
7.6" C, as compared with a difference of 16.8° during the preceding 
period. Furthermore, the mean date of the commencement of pregnancy 
was not May 2, but some days later. 
Fig. 5. 
24 . u 
23 /\ /\ > V ^ 23 
22 / \^^^ / \ X \ ..^-^^Nv 
20 / \ 20 
T Birth IsttMeas. ZdtMeas. 
May June July Auq. Sept. 
\^\5 1 8 -15 22 29 6 13 20 27 y 10 \1 24 31 T U 
Curve showing temperature conditions to which the offspring of the modified parents were subjected 
from biith up to the time of the last measurements. For further explanation see Fig. 1. 
From the time when pregnancy became apparent, the two sets 
of mothers were kept in the same room and under conditions which 
were indentical for all, so far as care could make them so. Unequal 
temperature conditions were particulary guarded against. The cages 
containing the two contrasted sets of animals were kept close together 
upon alternate shelves, and this arrangement was continued throughout 
the first six weeks of the life of the young. 
The temperature conditions to which the offspring were exposed 
are represented graphically in Fig. 5. The mean temperature for 
the entire period was 21.4" C. That from the mean date of birth 
(May 25) to the mean date of the first measurements (July 6) was 
20.8"; to the mean date of the second measurements (Sept. 8) it 
was 21.4". 
In all, about 182 living^) mice were born by the warm-room 
mothers ; about 172 by the cold-room mothers. Of these, 141 offspring 
^) A considerable number of still-born young are always to be reckoned with. 
