The cytoplasm of the sperm contains NOS (Nitric Oxide Synthase) which is activated along with the acrosome reaction by contact with egg jelly (yellow – NOS turns green here when activated). Activated NOS produces Nitric Oxide (light green spray).
Upon fusion with the egg, NOS is released into the egg, whereby the NO produced activates Calcium release (red spray). Calcium activates NOS already present in the egg, releasing more NO, which releases more Calcium, etc. Starting a wave of calcium throughout the egg.
Also see Acrosome Reaction.