Developmental changes of the hand skeleton induced by grafting leg mesoderm to the wing bud in the chicken.

In chick embryos of the stages 20 to 24, a fairly large portion of the intermediary region of the hind-limb was introduced into the wing bud from which a corresponding block of tissues had been excised. The graft was inserted with its axes coincident with those of the host bud, and it came in contact distally with a layer of apical mesoderm subjacent to the apical ridge leftin situ in the wing bud. According to the maps of the prospective territories, the graft contained part of the mesodermal territories of the stylo-zeugopod of the hind-limb; the layer of apical mesoderm leftin situ in the wing bud, distal to the graft, would have given origin to the hand.The grafted mesoderm showed a tendency to a gradual developmental reduction; apparently, it did not take a detectable part in the organogenesis of the host autopod. An anomalous cartilaginous nodule proximal to the wrist differentiated from the distal remnants of the graft; the rest of the graft took part possibly in the formation of the arm and forearm skeleton, which appeared normal or slightly reduced.The hand of the host wing showed a variety of quantitative and/or qualitative differences in the anatomy of its skeleton in comparison to the normal hand of the intact contralateral wing. These differences involved the part of the hand which apparently developed from the apical mesoderm of the host which lay distal to the graft. Cartilaginous pieces of normal shape but bigger than the controls, or variously modified as to their shape were observed consistently. In a significant percentage of embryos, the third ray of the hand showed morphological features more typical of a toe than of a hand-digit, or a supernumerary toe developed in association with the hand digits which appeared reduced to various degrees.The results are tentatively interpreted as the consequence of changes of the normal prospective pattern of development of the apical mesoderm of the host wing bud induced by the grafted mesoderm of the hind-limb. It is suggested that the mesoderm of the graft produced molecular materials which propagated to the overlying undifferentiated apical mesoderm of the host wing, and exerted inductor influences on the growth and on the further steps of development of the latter.