>> In human females oogenesis, or egg formation, takes place within the ovaries. Each ovary contains diploid cells called oogonia that are derived from embryonic germ cells. During fetal development in a female, the oogonia divide by mitosis. The result is more oogonia, some of which develop into primary oocytes. Primary oocytes are immature egg cells contained within masses of cells called follicles. The primary oocytes then enter meiosis I. The process arrests at this stage, and no more primary oocytes form after this point. Thus, at birth each female has a finite number of primary oocytes available for reproduction. At puberty, a complex series of hormonal events stimulates changes in the follicle cells that induce some primary oocytes to complete their first meiotic division. The division of cytoplasm and cell organelles is unequal, however. As a result, each primary oocyte forms one large secondary oocyte, which is haploid, and one small polar body, also haploid. In this way, most of the cellular cytoplasm remains in the developing oocyte. The polar body often degenerates. The follicle containing the secondary oocyte continues to mature until a surge of luteinizing hormone, or LH, initiates ovulation. The mature follicle ruptures, releasing the secondary oocyte into the oviduct, also called a fallopian tube. At ovulation, the secondary oocyte begins meiosis II. This second meiotic division will not be completed, however, unless sperm penetration occurs. The secondary oocyte enters the oviduct, where a sperm cell can penetrate it. The entry of a sperm cell into the cytoplasm of the secondary oocyte triggers the completion of meiosis II. Here, we show the chromosomes from the sperm at the left, while the chromosomes from the secondary oocyte are lined up in metaphase II. The cytoplasm divides unequally, generating a mature ovum and a second polar body. The fusion of the haploid sperm cell nucleus and the haploid egg cell nucleus, the process of fertilization, produces a diploid zygote.