Skeletal Development
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 tip of the backbone. By the seventh week it is about 2 cm (1 in) long and facial features are visible. At this stage, the 206 bones of the human body—tubular, round, and flat—are all set down, in surprisingly adult form. However, the process of osteogenesis—development of bone—has not progressed to the point where the bones will become "bony." Indeed, ossification—the process whereby tissue hardens completely and becomes bone—of most bony nuclei of the long bones and roundbones does not take place until after birth.
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