Each of these structures undergo a development and subsequent regression phase during the estrous cycle. Follicles begin within the ovary as one of several thousand primary follicles, which consist of a germ cell surrounded by a layer of flattened cells. This germ cell has the potential to mature into an egg if the follicle completes the development phase. However, only a small percentage of primary follicles continue through the secondary and tertiary follicular phases and ultimately undergo ovulation. Primary follicles that never complete development die and are replaced by newly formed primary follicles.