(1) The sex-ratio of over 1000 albino mice born in 157 litters worked out at 118 males per 100 females. In the records which I was able to secure of 255 other mice the ratio was 133.8.

Both these ratios are above other experimental determinations of the sex-ratio in mice.

(2) The seasonal variation in the new breeding records here presented is quite in keeping with work on other mammals, i.e. the ratio is lowest in the height of the breeding season and highest at the extreme end.

(3) The data relating to size of litter and sex-ratio show that abnormally small litters have a very low ratio compared with the average, and that abnormally large litters have a sex-ratio rather higher than the average.

(4) The sex-ratio is known in some cases to fall with each successive pregnancy, but in the present case the ratio of first births is lower than in subsequent ones.

(5) The interpretation of these facts, on the dual basis of the chromosome theory of sex determination and the sexually selective pre-natal mortality which is known to occur, is attempted.

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