It is a beautiful example of how the process of endochondral ossification proceeds from the center of a long bone towards its ends. The proximal and distal ends of the femurs,the humeri, and the radii, for example, have not yet established their local secondary centers of ossification, but the shaft of all these bones is well along in the process. The beginnings of ossification can be seen in the pelvic girdle as stippled red areas. The mandible is pretty well complete, the vertebrae are entirely collagenous.
Bone development is a dynamic and on-going process.
Even when the skeleton is fully matured and there is no more shaping and sculpting to be done, the removal and renewal of bone goes on. The necessity to maintain structural strength and physiological stability keeps bone an active, changing, growing tissue until the day the organism dies.