Oogenesis involves the development of oocyte and also folliculogenesis, which takes place in the Ovary. The primary oocyte develops first and begins its meiosis-I, which gets arrested at prophase-I. This stage is followed by the development of granulosa cells around it, known as primary follicle formation. The primary follicle then develops into the secondary follicle, which is characterized by the formation of the theca layer around granulosa cells. This then develops into the tertiary follicle. The differentiating feature is the formation of Antrum, which is initially crescent-shaped and later becomes large. This tertiary follicle still shows the presence of primary oocyte in it. These two then undergo further development. Primary oocyte completes its meiosis-I and forms two unequal cells: Large secondary oocyte and a small first polar body. Tertiary follicle further develops into Graffian follicle and secondary oocyte starts it meiosis-II.