Fertilization begins with the binding of head of the spermatozoan to the outer layer of the egg. Fusion with the first sperm initiates the first step of the cortical reaction, causing the egg to reverse electrical polarity, becoming positive intracellularly. In addition to preventing the immediate entrance of another sperm, the change in polarity causes the exit of calcium, sodium and hydrogen ions, which alters the pH, activating many enzymes and proteins. Thirdly, the fertilization membrane is elevated. Cortical granules fuse with the plasma membrane throughout the cell, releasing their contents, allowing the formation of the fertilization membrane.
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