Ca²⁺ oscillations and signaling represent a basic mechanism for controlling many cellular events. Activation of mouse eggs entrains a temporal series of Ca²⁺-dependent events that include cortical granule exocytosis, cell cycle resumption with concomitant decreases in MPF and MAP kinase activities, and recruitment of maternal mRNAs. The outcome is a switch in cellular differentiation, i.e., the conversion of the egg into the zygote. By activating mouse eggs with experimentally controlled and precisely defined Ca²⁺ transients, we demonstrate that each of these events is initiated by a different number of Ca²⁺ transients, while their completion requires a greater number of Ca²⁺ transients than for their initiation. This combination of differential responses to the number of Ca²⁺ transients provides strong evidence that a single Ca²⁺ transient-driven signaling system can initiate and drive a cell into a new developmental pathway, as well as can account for the temporal sequence of cellular changes associated with early development.