Oogonium (plural: oogonia) is a cell that arises from a primordial germ cells that will further differentiate into oocytes in the ovary. Oogonia are formed through mitosis. The formation starts to develop at the fourth week of the fetus development.
The blastocyst contains the primordial germ cells that come from the proximal epiblast. These germ cells will travel to the genital ridge until it reaches the undifferentiated gonads of the fetus. Primordial germ cells can move via amoeboid movement. During the fourth or fifth of development, the gonads will start to differentiate into ovaries (if Y chromosomes is absent, meaning the fetus is a female, XX). The ovaries continue to differentiate to form the cortical cords where the primordial germ cells will be collected.
During the sixth week of female embryonic development, the primordial germ cells grow and begin to differentiate into oogonia that will multiply through mitotic cell division.