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Light micrograph of endometrium (the mucosal lining of the uterus). If a fertilized ovum is not implanted, the endometrium degenerates and is shed. During this degeneration a high infiltration of white blood cells (leucocytes, purple) occurs. Under hormonal influences the endometrium undergoes regular cyclical changes so as to offer a suitable environment for implantation of a fertilized ovum. Magnification unknown. (KEYSTONE/SCIENCE SOURCE/BIOPHOTO ASSOC.)