A diploid cell formed after a diploid oogonium underwent mitosis. The primary oocyte is arrested in Prophase of Meiosis I. It will complete meiosis to form a haploid primary oocyte and the haploid first polar body.

During oogenesis, the immature egg cell in which the first meiotic division begins, leading to the production of the first polar body and the secondary oocyte.

The form of the ovum, or egg, produced in the ovaries of fetuses by oogonia that have begun the first part of the cell division known as meiosis (by which the chromosomes will eventually halve in number). Persists into childhood and adult life by containment in follicles. Gives rise to a secondary oocyte and the first polar body just before ovulation. See also: in vitro maturation