In the rabbit, daily sperm production by the testes was estimated by determining the total number of spermatozoa and late spermatids contained in a testicular suspension, and the length of the spermatogenic cycle was estimated by an autoradiographic technique. In twenty bucks, 9 to 14 months old (mean weight 3·9 kg), which had spent 6·9±1·8 weeks in a constant photoperiod of 14 hr light daily, the mean sperm production was 147·4 × 106/day and 1 g of their testicular tissue yielded 26·5 × 106 sperm/day. In another group of thirteen bucks, 15 to 16 months old, which had spent 26·3±1·7 weeks under similar lighting conditions, mean sperm production was 115·8 × 106/day and 1 g of testis yielded 21·4 × 106 sperm/day. The difference between the two groups of animals is highly significant and it is postulated that a constant photoperiod is deleterious for sperm production. In rabbits collected from twice a week the daily sperm output was consistently lower than the testicular sperm production, indicating that slightly more than 50% of the spermatozoa produced are resorbed.
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