Oogenesis is the process by which female gametes, or eggs, are produced in the ovaries. It begins before birth, with the formation of primary oocytes that remain dormant until puberty. Each month during the menstrual cycle, a few primary oocytes resume development, but typically only one matures into a secondary oocyte, ready for fertilization. During oogenesis, the primary oocyte undergoes meiosis, a type of cell division that reduces the chromosome number by half. This results in the formation of a secondary oocyte and a polar body, which is a small cell that usually degenerates. If fertilization occurs, the secondary oocyte completes meiosis, leading to the formation of a mature egg.