In recent reports from this laboratory, it has been shown that a series of complex events occur at the gamete surfaces after spermatozoa contact the egg (Hartmann, Gwatkin & Hutchison, 1972; Hartmann & Hutchison, 1973, 1974a, b). The relatively long period between the B1 and B2 bindings between the gamete surfaces suggests that changes occur which are a prelude to the penetration of the zona pellucida. Our earlier studies suggested that after pre-exposure to a population of eggs the spermatozoa were altered by the action of a vitelline factor in a manner which enabled them to bind sooner to a second population of eggs (Hartmann et al., 1972; Hartmann & Hutchison, 1974a). In this report, it will be shown that following B1 the zona pellucida is also altered in a manner which apparently prepares it for the final binding step (B2).
Spermatozoa were collected from the caudal portion of the