Sexual activity is more closely correlated with the weight of the testis than with the body-weight, since mature spermatozoa are found in almost all oo with testes weighing 100 mg. or more. This is a more accurate criterion of fecundity than the body-weight. Sexual activity commences early in March and is accompanied by an increase in body-weight and enormous growth of the testes from a winter weight of less than 40 mg. to a mean summer weight of 682 mg. Young [male][male] born early in the breeding season become sexually mature before the end of the same season; those born late in the season do not attain sexual maturity until the following spring. In late Aug., Sept., and Oct. the testes are in regression and by Nov. there is complete cessation of spermatogenetic activity.