During metII arrest, Emi2 is kept high. It inhibits the APC, thus preventing loss Cdk1 activity through cyclin B1 proteolysis. Cdk1 activity is further maintained by continual cyclin B1 synthesis and stimulation of Cdc25. MAPK levels are also high in unfertilized eggs. At fertilization, following sperm-egg fusion, PLCζ diffuses into the egg and hydrolyses PIP₂ to produce IP₃ which releases Ca²⁺ from the endoplasmic reticulum and so activates CamKII. Emi2 is a CamKII and Plk1 substrate, with its phosphorylation leading to degradation. Loss of Emi2 frees the APC to be active, causing cyclin B1 degradation and consequently lowering Cdk1 activity, a process enhanced by Cdk1 phosphorylation by Wee1b. APC activation also degrades securin leading to the activation of separase, promoting chromosome segregation by cohesin cleavage. The increase in cystosolic Ca²⁺ also decreases MAPK activity, which promotes the formation of the nuclear envelope.