(1) 1872 normal mice bred during November 1922-October 1925 had an average litter-size of 6.18, and a male percentage of 51.7 ± .77, both of which figures are lower than the corresponding ones found for 1921-2.

(2) The annual variation in the sex-ratio is, however, not very appreciable, the figures for the four years running 54.2 ± 1.04, 50.4 ± 3.22, 52.2 ± 1.18, 51.4 ± 1.09. The average size of litter during these four periods ranged from 6.65 to 5.93.

(3) Considerable seasonal variation in both fertility and sex-ratio occurred, the highest figures being fertility 6.46 and male percentage 55.9 ± 1.83 in the October-December quarter, and the lowest being fertility 5.82 and male percentage 48.2 ± 1.46 in the April-June period.

(4) Litter-size appeared to be uncorrelated with any sort of definite variation in the sex-ratio.

(5) Recent work on the mouse is discussed.

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