[Morphogenesis of the chick embryo limb. Competence of the embryonic and extra-embryonic ectoderm]

Ecto-mesodermal interactions were investigated during the initiation of limb development in avian embryos. Experiments were performed on 2-day chick embryos. They consisted in implanting prospective leg mesoderm at different medio-lateral levels of the trunk and also into the extra-embryonic area. The implanted mesoderm was thus brought into contact with embryonic or extra-embryonic cicatricial or healing ectoderm, the ability of which to participate in the formation of an ectopic leg was tested. Whatever the level of embryonic ectoderm tested in hosts ranging from stage 14 to 27 pairs of somites (axial, paraxial, flank, ventrum), the experiments resulted in the formation of supernumerary limbs. Their frequency was level-dependent and decreased for each level, with increasing age of the host. The weakest competence was observed in the ectoderm of the prospective ventrum, the strongest in that of the prospective flank, axial and paraxial ectoderm showing an intermediary competence. Extra-embryonic ectoderm of blastoderms of the same age was unable to respond to the inducing action of the implanted prospective leg mesoderm. It was found to be incompetent, even at younger stages (5 to 13 pairs of somites).