FERTILIZATION, DRAWING

Ovum and spermatozoid. Oogenesis has three phases, multiplication (to about 300,000 primary ovocytes), growth and maturation (of about 300 - 400 ovocytes). During the maturation phase, primary oocytes undergo division (meiosis) becoming secondary oocytes which expulse a polar body (lower left). A second division then starts but stops at the metaphase (lower middle). If the ovum is fertilized (penetration of a single spermatozoid), the cell division is completed (lower left). If the oocyte is not fertilized it degenerates more or less rapidly