Evidence for a metameric pattern in the development of the chick humerus

It has been proposed that the wing bud is induced by some axial influence at a specific confined location and that the ZPA is the residual influence of such induction. The purpose of the present investigation was to test this hypothesis. Tantalum foil barriers were placed lateral to the mesonephric duct and parallel to the long axis of the embryo in the wing field of stage-12 to -15 chick embryos. These barriers blocked the somatopleure's communication with more medial tissues at specific somitic levels. The results of these experiments demonstrate that (1) the limb is not induced at one specific point, (2) portions of the humerus appear to be induced segmentally along the entire limb field and (3) the ZPA is not induced by axial structures. We propose a model of wing development suggesting that the humerus is induced as several separate components which then fuse to form the definitive bone.