866 
J. FRANK DANIEL 
In a consideration of the subject of gestation mice fall natur- 
ally into two well defined groups: (1) those that suckle and (2) 
those that do not suckle young during pregnancy. The latter 
class of non-suckling mice includes (a) all cases of females 
carrying their first litter, and (b) those cases in which copulation 
took place after the young were weaned, or those cases in which 
copulation took place and the young were separated from the 
mother or were early lost. For all such, a definite gestation 
period of practically 20 days may be stated ; and this holds true 
irrespective of size of litter born. 
While 20 days may be considered as the average period for non- 
suckling mothers, for suckling mothers the period presents no such 
regularity. This will be made evident from table 1, in which ten 
consecutive cases are considered. 
TABLE 1 
EXAMPLE 
DESIGNATION OF 
FEMALE 
NUMBER OF YOUNG 
SUCKLED 
PERIOD OF GESTATION 
NUMBER BORN 
1 
F 
10 
30 days 
9 
2 
M 
6 
24 days 
6 
3 
J 
8 
27 days 
7 
4 
L 
3 
25 days 
5 
5 
M 
6 
24 days 19 hours 
11 
6 
H 
3 
22 days 
5 
7 
M 
10 
30 days 3 hours. 
9 
8 
H 
5 
25 days 
1 
9 
J 
5 
24 days 14 hours 
7 . 
10 
F 
8 
27 days 15 hours 
8 
In table 1 two things stand out prominently : First the marked 
variability, and second the lengthening, of the period of gesta- 
tion. From the table it will be seen that not only does one mother 
differ from another in period of gestation, but the same mother at 
different times shows a similar variability. As an example of the 
two cases: H (example 6) went but 22 days from the time of 
copulation to the birth of her young, while M at one time had a 
period of 30 days + (example 7). The same mouse (M) at another 
time, however, had a period of only 24 days (example 2) . 
