In females, meiosis occurs in cells known as oocytes (singular&colon; oocyte). The zona pellucida is essential for oocyte growth and fertilization. It results from cleavage of the oocyte (ovum) after fertilization. Each of them contains a single oocyte (immature ovum or egg cell). The oocyte completes its second meiotic division. The first being a female gamete, known as the ovum (egg/oocyte). Acrosin digests the zona pellucida and membrane of the oocyte. Removal of the oocyte from the follicle causes meiosis to progress in the oocyte. Sperm entry into the oocyte commences formation of an anterior-posterior axis. The primary oocyte turns into a secondary oocyte in mature ovarian follicles. Laboratories have developed grading methods to judge oocyte and embryo quality. The follicle and the ovary's wall rupture, allowing the secondary oocyte to escape. The cortical reaction occurs due to calcium oscillations inside the oocyte. The fusion of cell membranes of the secondary oocyte and sperm takes place. These follicles each contain a similarly immature primary oocyte.