The primary follicle, which is formed when each primary oocyte is encircled by a layer of granulosa cells, is subsequently formed. During the period from infancy to puberty, many of these follicles degenerates. As a result, each ovary only contains primary follicles at puberty. The secondary follicles are formed when the primary follicles are encircled by additional layers of granulosa cells and a new theca. The tertiary follicle is distinguished by a cavity filled with fluid called the antrum.

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