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Bird eggs and embryo. X-ray of broken egg shells after hatching, and an egg with a well-developed bird embryo at upper right. The X-ray shows the bones of its skeleton, including the vertebrae of the backbone and the wing and leg bones. Typical incubation periods for eggs such as those laid by chickens are around 20 days, though in other species the incubation period can range from 10 to 45 days. (KEYSTONE/SCIENCE PHOTO LIBRARY/GUSTOIMAGES/SCIENCE PHOTO LIBRARY)