1. In the ovaries of a female fetus, cells called oogonia (2n) divide by mitosis to make more oogonia cells.
2. Oogonia grow into larger cells called primary oocytes (2n).
3. Primary oocytes start meiosis but stop at prophase I. Primary oocytes and a single layer of follicle cells form a primary follicle.
4. When a baby girl is born, the ovaries contain 400,000 primary follicles.
5. Every menstrual cycle, a few primary follicles start to develop -- During this time, the Primary oocyte completes meiosis I, forming two haploid (n) daughter nuclei - cytoplasm of the primary oocyte develops unequally, forming a secondary oocyte (n) and a small polar body (n).
6. Then, the Secondary oocyte starts second division of meiosis but stops at prophase II. The follicle cells, meanwhile, are reproducing quickly and follicle fluid is forming.
7. When the mature follicle bursts at the time of ovulation, the released egg is still a secondary oocyte (n).
8. After fertilization, the secondary oocyte completes the second division of meiosis to form an ovum with the sperm nucleus already inside.
* Polar bodies do not develop into eggs; they degenerate.