In this photograph, the chorion (the shaggy outer membrane surrounding a fetus) is folded back to reveal the human fetus through the thin amniotic membrane. At around 12 weeks the fetus is about 5 centimeters long. The head, arms and legs, and even fingers and toes, are clearly visible. It is also possible to make out the ears, eyes and ribs. At this stage, the first bone tissue is developing, replacing the initial cartilage skeleton.