<P> By this time, the oocyte has completed meiosis I, yielding two cells: the larger secondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body . Meiosis II follows at once but will be arrested in the metaphase and will so remain until fertilization . The spindle apparatus of the second meiotic division appears at the time of ovulation . If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation . Approximately 1 - 2% of ovulations release more than one oocyte . This tendency increases with maternal age . Fertilization of two different oocytes by two different spermatozoa results in fraternal twins . </P> <P> The mucous membrane of the uterus, termed the functionalis, has reached its maximum size, and so have the endometrial glands, although they are still non-secretory . </P> <P> The follicle proper has met the end of its lifespan . Without the oocyte, the follicle folds inward on itself, transforming into the corpus luteum (pl . corpora lutea), a steroidogenic cluster of cells that produces estrogen and progesterone . These hormones induce the endometrial glands to begin production of the proliferative endometrium and later into secretory endometrium, the site of embryonic growth if implantation occurs . The action of progesterone increases basal body temperature by one - quarter to one - half degree Celsius (one - half to one degree Fahrenheit). The corpus luteum continues this paracrine action for the remainder of the menstrual cycle, maintaining the endometrium, before disintegrating into scar tissue during menses . </P> <P> The start of ovulation can be detected by signs . Because the signs are not readily discernible by people other than the female, humans are said to have a concealed ovulation . In many animal species there are distinctive signals indicating the period when the female is fertile . Several explanations have been proposed to explain concealed ovulation in humans . </P>

Immediately after expulsion of the ovum the ruptured follicle is now referred to as