An oocyte is a cell in the ovary that undergoes meiosis to form an ovum. Primary oocytes develop from oogonia in the fetal ovary as they enter the early stages of meiosis. Only a fraction of the primary oocytes survive until puberty, and even fewer will be ovulated. At ovulation the first meiotic division is completed and a secondary oocyte and a polar body are formed. Fertilization stimulates the completion of the second meiotic division, which produces a second polar body and an ovum.
The nucleus of a mature oocyte, prior to fertilization, is called a germinal vesicle. It is considerably larger than the nucleus of other cells.