Since 1955, we have bred the Japanese mouse, Mus molossinus Temminck and Schlegel in our laboratory. During the period, basing on thirty seven litters, litter size, number of the weanling per litter and sex ratio are observed. Main results are as follows : 1 . The average litter size is 5.32 ± 0.09, and the sex ratio is 97.00 : 100 at birth (Table 2 ). 2 . In these data, seasonal variations in litter size and sex ratio have not been clarified yet. But judging from eighty one pregnancies observed under natural environments, we can suggest that seasonal variation may be caused by atrophic foetuses in some of the reared mice (Table 1 ). 3 . The average lactating period seems to be 17.30 ± 0.177 days (Table 4 ). 4 . The mean number of the weanling per litter is 4.70 ± 0.124, and the sex ratio is 89.13 : 100, which does not differ significantly from other related cases (Table 4 ).
Mendeley出力
