If a fragment of uterine mucous membrane (endometrium) is placed directly on a sample of peritoneum in a laboratory experiment, examination under the microscope reveals that this endometrium adheres to the peritoneum within the space of an hour. After 18 hours endometrial cells have already partly the peritoneal tissue. Endometrial cells have this capacity in the body, too, and they can develop deep within tissues. To feed themselves, the islands of mucous membrane developed in this way have to form their own blood vessels as soon as their volume exceeds 3 mm³. The formation of these new vessels is accompanied by inflammation. which can be detected in all cases of endometriosis.