A follicle is a vesicle filled with liquid which contains an egg. After birth, the ovary contains about 1.5 million follicles, but only about 400 follicles survive until the stage of maturity, when an egg capable of being fertilized is released.

Follicles are approximately 8 mm long but they keep growing until they are 18-24 mm long. Then the follicle breaks open and the egg is released.

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