The cortical granule reaction requires an increase in intracellular:
Cortical granule exocytosis is triggered by calcium wave sweeping from sperm entry point across cortex. Phospholipase C zeta introduced by sperm hydrolyzes PIP2 generating IP3, which binds receptors on endoplasmic reticulum releasing stored Ca2+ into cytosol. Rising Ca2+ binds synaptotagmin and triggers SNARE-mediated fusion of cortical granules with plasma membrane. Calcium ionophore A23187 can artificially activate eggs mimicking envelope elevation. Sodium, potassium and magnesium fluctuations follow depolarization but essential fusogenic trigger is intracellular Ca2+ rise linking fertilization to polyspermy block and metabolic activation.
Ref: Whitaker, Physiol Rev 2006, Calcium wave in egg activation; Gilbert Chapter 7: Ca2+ dependent cortical reaction.