Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545, U.S.A.
In a study of the penetration of zona-free eggs by foreign spermatozoa in the mouse, rat and hamster, it was concluded that the incorporation of eggs with foreign spermatozoa may depend on sperm capacitation and the physiological affinity between the spermatozoa and the vitellus of different species, and that the zona pellucida appears to be the major block for interspecific fertilization (Hanada & Chang, 1972). Yanagimachi (1972) has reported that hamster zona-free eggs can be penetrated readily by capacitated but not by uncapacitated guinea-pig spermatozoa. Based upon our recent studies of sperm capacitation and fertilization of rat eggs in vitro (Toyoda & Chang, 1974; Niwa & Chang, 1974, 1975), this paper describes the penetration of hamster and rabbit zona-free eggs by mouse and rat spermatozoa as affected by the capacitation of spermatozoa.
The medium used for the manipulation of gametes was
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