The mechanisms for the prevention of polyspermy in mammalian eggs appear to be associated with the development of the so-called `zona reaction' (Braden, Austin & David, 1954) and with the development of a vitelline surface block to polyspermy (Austin & Braden, 1956). The time required for the development of the zona reaction has been estimated to be not less than 10 min and not more than 1½ to 2 hr (Braden et al., 1954).
Hamster eggs recovered from mated females show a low incidence (1·6%) of polyspermy (Austin & Braden, 1956) while eggs fertilized in vitro show a high incidence, often as high as 100% (Yanagimachi & Chang, 1964; Barros & Austin, 1967; Barros, 1968b; Yanagimachi, 1969).
It has not been established whether the high incidence of polyspermy in vitro is due to the penetration of several spermatozoa at the same time or to a continuous flow of spermatozoa
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