It has been reported that rat eggs can be fertilized in vitro (Miyamoto & Chang, 1973; Toyoda & Chang, 1974). Further study has revealed that the eggs from immature rats are easier to fertilize in vitro with a lower concentration of spermatozoa (Niwa & Chang, 1973) and that the capacitation of rat spermatozoa can be better achieved in a lower concentration than in a higher concentration of spermatozoa (Niwa & Chang, 1974). The present experiment was designed to determine the optimal sperm concentration and the minimal number of spermatozoa for the fertilization of rat eggs in vitro.
The medium for fertilization was prepared according to the description of Toyoda & Chang (1974). The eggs recovered from naturally ovulating mature rats or from immature rats which had been induced to superovulate were inseminated with epididymal spermatozoa from mature rats, incubated and examined as described by Niwa & Chang (1973). In order
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