Oogenesis (formation of the ovum)

Three stages:

1. Multiplication phase: mitosis of the primary germ cells (2N) → oogonia (2N); this happens in the embryo (before birth).
2. Growth phase: the oogonia store food, grow, and become primary oocytes (2N), also in the embryo; they then stay at the prophase of meiosis I until puberty.
3. Maturation phase: at puberty, once a month, a primary oocyte completes meiosis I forming a large secondary oocyte (N) and a small first polar body (N). The secondary oocyte starts meiosis II and is released at ovulation; it completes meiosis II only if a sperm enters, giving the ovum (N) and a second polar body. The first polar body may also divide, so up to three polar bodies form, which later degenerate.

Result: one primary oocyte → 1 ovum + 3 polar bodies (unequal division gives the egg most of the cytoplasm and food).

The ovum is surrounded by a thin cellular coat held together by hyaluronic acid; the enzyme from the acrosomes of many sperms dissolves it at the point of penetration — so millions of sperms are needed for one to reach and penetrate the egg.