Summary. Blood vascular casts of rat ovaries were prepared by injection of a low viscosity methacrylate medium, and observed by scanning electron microscopy. The proper vascular pattern of the rat ovarian follicle starts as a basket-like wreath of fine capillaries around the primary follicle. As the follicle grows larger, the wreath becomes more developed. The fully developed wreath around the Graafian follicle consists of sinusoidal capillaries. In ovulation, the apical area of the wreath is opened and the basal area of the wreath is elevated. After ovulation, the wreath shrinks and its opened area is closed, with markedly dilated vessels directly continuous with the arterial capillaries. The wreath then transforms, by intense new formation of capillaries, into a conglomerated sinusoidal capillary plexus with an avascular area within it. The avascular area soon disappears by continued intense formation of capillaries in the plexus, and a complete and dense vascular network of the corpus luteum is formed. The capillaries of the fully developed corpus luteum are small in caliber and not sinusoidal in nature. Discontinuities of the capillaries and a flattening of the efferent branches are the initial signs of the degeneration of the corpus luteum. The small markedly degenerated corpus luteum contains only scattered thready capillaries. The corpus albicans contains few blood vessels.

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