a1 Istituto di Fisiologia Veterinaria-Ozzano Emilia, Bologna, Italy
Before fertilisation the egg is metabolically quiescent and its nucleus is arrested at metaphase of the second meiosis. After sperm-egg fusion, the arrested nucleus resumes meiosis, and then changes into the female pronucleus. Such a sequence of morphological and biochemical events is called “activation”. The inital responses of the egg to activation by the sperm include cortical granule exocytosis and resumption of meiosis.