One secondary follicle has reached maturity. It is called a Graafian follicle. Facing this follicle, the albuginea is lysed at the point of the stigma. Through the small hole thus formed, the oocyte, surrounded by its cumulus oophorus, and the follicular fluid are released into the abdominal cavity.

The residual follicle collapses, and its wall,which is made of follicular or granulosa cells and cells of the theca interna, takes a wavy appearance, and rapidly transforms into a corpus luteum.

The granulosa cells enlarge, become spheroidal and secrete progesterone. They are called granulosa lutein cells. The cells of the theca interna also become steroid-secreting cells that become cuboidal but remain small. They are referred to as theca lutein cells. They form cords in the crevices formed by the granulosa lutein cells.

The blood vessels surrounding this collapsed follicle grow rapidly and invade the developing corpus luteum. This corpus luteum is an endocrine organ which has a transitory existence. It eventually regresses and forms a large fibrous connective tissue mass called the corpus albicans.