Magnified view of an ovulating follicle in cross-section.

As the follicle opens (at the "stigma") to release the egg -- which is surrounded by its sticky, mucus-like "cumulus" -- hormones such as estrogen and progesterone (which are made by the lining of the follicle) pour out into the peritoneal fluid. This is the same peritoneal fluid that also bathes patches of endometriosis. The amounts of hormone reaching the endometriosis this way are hundreds of times higher than reach it through the blood.

This is the ovary of a mouse, but a human follicle looks and ovulates the same way.

Credit: Dr Richard Blandau