HA4 shows a secondary follicle. The liquor folliculi (follicular fluid) in the antrum (a) distinguishes this secondary follicle from the late primary follicle in HA3. Identify the oocyte, zona pellucida, and membrana granulosa. Note that two layers have formed around the follicle. These are the theca interna (in), which stains differently than the theca externa (ex), due to its rich vascular supply.