Illustration of a human embryo about 13 days post-conception embedded in the wall of the uterus. The embryo is a bilaminar disc with amnion and developing yolk sac above and below respectively. A stalk connects the embryo to the early placental tissues. Uterine glands are seen between maternal blood vessels (red). From Gray, H, (1918) Anatomy descriptive and applied. 20th ed. Longmans Green, London.