Final answer:
A primary follicle enlarges and develops into a secondary follicle, which then grows into a tertiary follicle or antral follicle once fluid-filled vesicles form and accumulate into an antrum.
Step-by-step explanation:
When a primary follicle enlarges and fluid-filled vesicles, known as follicular fluid, develop among the several layers of granulosa cells, it becomes a secondary follicle. The growth process involves the primary oocyte secreting a membrane called the zona pellucida and follicular fluid collecting into a large pool called an antrum. When the antrum is fully formed, the follicle is then known as a tertiary follicle, or antral follicle. Most tertiary follicles undergo atresia, but the one that survives continues to develop until ovulation when it expels its oocyte.