At 40x the very large, dense, pink, corpus luteum (Latin yellow body) of ovulation dominates the field of view. The corpus luteum produces estrogen and progesterone in the luteal phase of the ovary. If pregnancy occurs these reach such large proportions that they dominate the ovary and little else can be distinguished.  They produce large reddish orange "pimples" which can be easily distinguished on the ovaries of pregnant animals, or those who have been given HCG, a hormone present in the urine of pregnant women. Observation of corpora lutea in rabbits given urine extract from a pregnant woman served as the pregnancy test for many years prior to the development of immuno-assay for HCG.