Due to more secretion of $\mathrm{LH}$ in the middle of menstrual cycle, Graafian follicle breaks and due to that ovum is released at this time remaining parts of graafian follicle are transformed into corpus leuteum. Corpus leuteum secretes more amount of projesterone which is necessary for maintenance of endometrium. For implantation of such endometrium fertilized ovum and remaining processes of conceiving.