During the process of oogenesis in the ovary, follicles go through several stages of development. The secondary follicle is characterized by the appearance of a specific fluid-filled space:

• The Antrum: As the secondary follicle matures, a fluid-filled cavity called the antrum begins to form. This cavity is filled with follicular fluid (liquor folliculi), which contains nutrients and hormones essential for the developing oocyte.

• Theca Layer: The secondary follicle also organizes its surrounding cells into an inner theca interna and an outer theca externa.

• Zona Pellucida: This is a transparent membrane that surrounds the primary oocyte, but it is not the cavity itself.

• Corpus Luteum: This is the structure formed from the remnants of the Graafian follicle after ovulation has occurred.