At fertilization, mammalian eggs show repetitive transient [Ca²⁺]_i rises each of which is due to Ca²⁺ release from the endoplasmic reticulum through inositol 1,4,5-trisphosphate (IP₃) receptors. During fertilization, a factor from the sperm, the sperm factor, is released into the oocyte and induces a long-lasting series of Ca²⁺ spikes (Ca²⁺ oscillations) that are required for egg activation. IP₃-producing enzyme phospholipase C zeta (PLC) is a strong candidate for the sperm factor. The Ca²⁺ spikes initiate the extrusion of cortical granules that block the entry of other sperm. At the same time, maturation (M-phase) promoting factor (MPF) is inactivated by the Ca²⁺ oscillations, resulting in exit from metaphase II arrest. Meiosis resumes with formation of the second polar body and complete meiotic division, one-cell embryos with the male and female pronuclei attain the first cleavage division through nuclear envelope breakdown.