Ovary in Oestrus

During oestrus, the follicle reaches its maximum size very quickly, growing to a diameter of 1.5 to 2 cm. The follicle contains a fluid that surrounds and cushions the egg. The yellow body (corpus luteum) from the previous cycle may still be present. It can be on the same ovary as the follicle, or on the other ovary.
In exceptional cases, two follicles can mature at the same time. Both follicles can be on the same ovary, or one follicle can mature on each ovary. After a successful insemination, the result is non-identical (dizygotic) twins. Identical (monozygotic) twins are born if the fertilised egg divides at an early stage of development.