The post-ovulatory ovarian follicle of the hen was studied histologically for regressive changes. The results obtained are as follows.
1. After ovulation the follicle shrank to a fiat thin-walled sac with an opening at the stigma. The ruptured follicle underwent progressive atrophy and disappeared ultimately into the stroma of the ovary.
2. After ovulation the empty follicular cavity was filled with an aggregative mass of vacuolated cells, the vacuoles of which were strongly sudanophilic. The vacuolated cells were derived from epithelial cells, follicular cells located in the internal theca, and fibrocytes present in the internal and external theca.
3. The basement membrane was detached from the internal theca at the time of ovulation. It thickened and wrinkled extensively after ovulation. It was rich in mucopolysaccharides.
4. Varying degrees of hemorrhage occurred in the inner wall of the follicle at the time of ovulation. Proliferation of capillaries was presented in the injured area of the inner wall of the follicle.
5. The blood vessels of the follicular wall underwent gradual atrophy after ovulation. In the course of regression, a large quantity of mucopolysaccharides was demonstrated in the vascular wall.