The sequence of oogenesis (formation of ovum) is as follows:

Millions of oogonia are added in a female fetal ovary after birth. These oogonia (2n) undergo division and lead to the formation of primary oocytes (2n). A layer of granulosa cells surrounds these oocytes, which leads to the formation of primary follicles. More and more layer of granulosa cells and a new theca surround these primary follicles which leads to the formation of secondary follicles A fluid filled cavity called antrum develops which leads to the formation of tertiary follicle. An unequal division of the primary oocyte in the tertiary follicle leads to the formation of a haploid secondary oocyte i.e. ovum and a tiny first polar body. The largets follicle formed is Graafian follicle. The ovum is released from the Graafian follicle in the secondary oocyte stage