After ovulation many of the follicular cells
remain in the collapsed graafian follicle on the
surface of the ovary. The antrum of the collapsed follicle
fills with a partially clotted fluid.
The follicular cells enlarge and fill with a yellow pigment,
lutein. Such a follicle is called a corpus luteum
which is literally, yellow body.
These remnants of the follicle becomes the
corpus luteum (yellow body), that produces progesterone to prepare
the lining of the uterus for implantation and pregnancy.