In early fetal development, oogonia undergo numerous mitotic divisions. At birth, a woman's ovaries hold approximately one million primary oocytes. The primary oocytes begin to divide by meiosis before birth, replicating their DNA, but the cells enter the state of meiotic arrest. Having formed sister chromatids, the meiotic division is not completed until ovulation much later in puberty. During ovulation the meiotic divisions occur, but the two daughter cells of the first division are not equivalent. The secondary oocyte inherits nearly all the cytoplasm, with the other daughter cell, the first polar body, adhering to the secondary oocyte. The second meiotic division, in which the sister chromatids separate, occurs only after secondary oocyte is fertilized. One daughter cell again receives almost all the cytoplasm, the mature ovum, with the second polar body, the other daughter cell, being small and nonfunctional.
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