Development of the bones of the human body.

Scientists who study the development of the human being from conception to birth begin calling the embryo a fetus around eight weeks after conception, when the first bone cells appear. The beginnings of the skeletal system begins prior to this, however. In the third week after conception, the notochord—a rod-like structure along the back of the embryo that will later become the spine, spinal cord, and brain—develops, followed in the fourth week by the first signs of arms and legs. Between the fifth and eighth weeks, the limbs (first the arms, hands, and fingers, followed by the legs, feet, and toes) begin to extend and take on a definite shape. By the end of the fifth week, the embryo has doubled in size and has grown a tail-like structure that will become the coccyx, or lowermost...