The series of drawings around it shows the main steps of oogenesis and follicular development, as indicated by the blue arrows:

1. Primordial follicle
2. Primary unilaminar follicle (with a single layer of follicular cells)
3. Primary multilaminar follicle (showing several layers of follicular cells)
4. Secondary antral follicle (i.e., showing a cavity or antrum containing follicular fluid)
5. Mature or preovulatory follicle

The mature or preovulatory follicle, also called a Graafian follicle, precedes ovulation. The sizes of the various drawings of the follicles are not representative of their actual relative sizes. Thus, for example, the diameter of a primary unilamellar follicle is about 50 µm, while the diameter of a mature or Graafian follicle is close to 1000 µm, or 20 times larger than the diameter of a primary unilamellar follicle.