Chicken embryo development. Coloured X-rays of the development of a chicken (Gallus sp.) in an egg. At left is the egg after about six days, showing only the air space (dark, at bottom) through which the developing chick will breath. At centre left is the egg after 12 days. The development of the chick's bones (green) has begun. The chick's skull is seen just above the left edge of the air space, and its neck and backbone are seen extending around to the opposite side of the egg. At centre right is the egg after 18 days. Wing and leg bones (upper part of egg) are more fully developed, and some blood vessels are seen. The hatched chick, after 21 days, is at right.