Illustration of a section through the endometrium (uterine lining). The endometrium is a membrane that is rich in blood vessels and glands. During the menstrual cycle it increases in thickness in preparation for the implantation of a fertilised egg. If at the end of the cycle an egg has not implanted the endometrium is shed during menstruation (period). Tubular glands in the endometrium secrete mucus during this stage to moisten the uterus. From Traite d'Anatomie Humaine, Vol 4 (1905) by French anatomist Leo Testut (1849-1925).