female germ cell; formed in the ovarian follicle where it accumulates yolk materials from the liver as it approaches ovulation into the oviduct; in the ovary the oocyte and yolk are surrounded by 2 thin membranes, the vitelline membrane (oolemma) and the perivitelline membrane; ovulation occurs when the follicle ruptures to allow the oocyte and its yolk to enter the infundibulum of the oviduct, where it can be fertilized by sperm