(a) 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.