1. Chromosome dimorphism of the spermatozoa has been shown for a variety of mammals, and in some cases this has been shown to be correlated with dimorphism in the head lengths of the spermatozoa.

2. In the present paper this correlation has been extended to the spermatozoa of man, the mouse, and the rat, in which chromosome dimorphism of the spermatozoa had previously been shown, and in which head length dimorphism seems to exist.

3. The interest of these results lies in the probability that the histological difference in the X- and Y-spermatozoa may account for the inequality of the sexes at conception in mammals.

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