The anatomy of the reproductive organs of female plains viscacha was examined at various stages of the reproductive cycle. The ovary is peculiarly flat with deep and convoluted surface invaginations. The ripe follicle is 200 μm in diameter and spontaneous ovulation of up to 800 eggs at a time takes place. True and accessory corpora lutea may be formed but it is suggested that luteinization occurs only after copulation. About seven ova are fertilized and implant, but resorption of all except the most distally situated foetus in each uterine horn occurs by Day 90 of pregnancy. No changes were detected in the uterine histology at different stages of the cycle. The cervix is not muscular but there is a longitudinal vaginal septum which extends distal to the cervical openings, causing the copulatory plug which is formed at mating to be Y-shaped. The vaginal epithelium shows changes associated with ovarian activity.
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