In egg-laying amniotes (reptiles, birds and monotremes), the developing embryo is tethered to a number of the extraembryonic membranes that deliver oxygen and nutrients and remove metabolic waste products throughout embryonic development. Prior to, or soon after hatching, these membranes detach from the animal leaving a temporary or permanent umbilical scar equivalent to the navel or ‘belly button’ in some placental mammals, including humans....