PERIOD OF GESTATION IN WHITE MICE 
867 
Equally as striking is the lengthening of the period. It is seen 
that the minimum period of 22 days in the mice under consider- 
ation is longer than the period (20 days) given for mice of group 1, 
while the maximum period in this group sometimes exceeds by one- 
third the period in group 1. 
It is thus evident that for group (2) with variations running 
from two to ten days, no definite time as a period of gestation 
can be stated. This does not imply, however, that the period of 
gestation is not orderly. On the contrary, the variability which in 
table 1 is so evident, gives place, upon a rearrangement of the 
data, to striking uniformity. 
What is the factor upon which this rearrangement can be made? 
From cases of prolonged gestation like 1 and 7, from which 
large numbers of young resulted, my first impression was that 
the number horn might be a factor in this variability. That this 
is not the case, however, can be shown conclusively by rearranging 
the table in order of the number born. In such a rearrangement 
it is seen that H (example 8), which gave birth to the smallest 
number, went longer than M (example 5), which gave birth to the 
greatest number. 
If on the other hand the number suckled rather than the number 
born govern the variability in the period of gestation, it would be 
expected that, given a definite number of suckling young, the 
period would be practically the same in all cases. This, in fact, 
is seen to be the case. For mouse M suckling 6 at two different 
times (examples 2 and 5) had in one case a period of 24 days and 
in the other of 24 da3^s and 19 hours. J and F (examples 3 and 10) 
suckling 8 each, went 27 days, and 27 days 15 hours, respectively. 
F and M (examples 1 and 7) suckling 10 went 30 days and 30 
days 3 hours, respectivel3^ 
Moreover, this order is so definite that a series increasing from 
the minimum to the maximum variation may be constructed thus : 
H suckling 3 went 22 days (example 6), J suckling 5 went 24 
days + (example 9), F suckling 8 went 27 days + (example 10), 
and M suckling 10 went 30 days -f (example 7). 
This sequence may be shown to further advantage by rearrang- 
ing table 1 in order of the number suckled. Such a rearrangement 
is made in table 2. 
