Ovulation in the dog may occur over a period of 12 to 24 hours, thus follicles and corpora hemorrhagica may be present at the same time. The ovary on the left shows a small protruding corpus hemorrhagicum and a medium size follicle, while the ovary on the right shows an ovulatory follicle. This occurs in other polytocous species (e.g. cats, pigs) and even in mares, where "asynchronous" ovulations (ovulations 12-24 hours apart)may occur.
Shille VM (1975)