(c) : After ovulation which occurs in the middle of menstrual cycle, empty Graafian follicle continues growth under the influence of $LH$. The follicular cells are converted into lutein cells by deposition of yellowish lipid inclusions. The phenomenon is called luteinization. The ruptured Graafian follicle is now called corpus luteum. It secretes hormones, mainly progesterone and small quantity of estrogen. Both $LH$ and progesterone help in further growth and thickening of endometrium. The major change is that the endometrial glands become secretory. The uterine wall becomes ready for nourishing and anchoring blastocyst if fertilization takes place. Hence, this phase of menstrual cycle is called luteal or secretory phase. The phase lasts for about $13$ days $i.e.$, $15-­28$ days of $28$ days menstrual cycle.