1. The average size of 267 litters of albino mice totalling 1795 animals was 6.72 ; in addition, the distribution of fertility between the two cornua of the uterus was found to be very even.

2. Seasonal variation in fertility was found to be slightly in favour of the summer months. Without artificially maintaining an optimum temperature, the mice were found not to breed in the months November to February.

3. Of consecutive pregnancies the second was found to produce the largest number of young, while concurrent lactation was found to decrease the size of litter, as also, to a lesser degree, did polygyny.

4. The interpretation of these results is attempted in the light of the two factors which govern the size of litter at birth, i.e. the number of ova ovulated and the amount of prenatal mortality.

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