Ovulated eggs from mature rats and superovulated eggs from immature rats were incubated with various concentrations of epididymal spermatozoa from mature rats. Few eggs from mature rats and only 16 to 20% of the superovulated eggs from immature rats were fertilized when the sperm concentration was 1·8 to 3·7 × 106 spermatozoa/ml. At concentrations of 0·7 to 1·5 × 106 spermatozoa/ml, 2 to 8% of the eggs from mature rats and 96% of the eggs from immature rats were fertilized. It appears that the eggs from the immature rats are much easier to fertilize in vitro than those from the mature rats, but the optimal concentration of spermatozoa also plays an important rôle.
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