The regional embryonic origin of trunk and limb musculature was determined through heterospecific homotopic or heterotopic transplantations of quail somitic or somiticsomatopleural mesoderm into chick hosts, and through localized X-irradiation of the somitic mesoderm. Experiments were performed on 2-day embryos. Results show that the myoblastic component of all truncal and appendicular muscles is of somitic origin. Intra- and perimuscular connective tissue as well as tendons are of somatopleural origin. X-ray destruction of the somitic mesoderm at and beyond the wing level resulted in the complete or almost complete absence of musculature in the wing and corresponding truncal region. The mapping of the cephalocaudal origin of the various muscles was found to be as follows: Intrinsic and extrinsic muscles of the wing and scapular girdle Somites 12-20 Grand pectoral muscle Somites 12-22 Intercostal muscles Somites 19-26 Abdominal muscles Somites 27-29 Intrinsic and extrinsic muscles of the leg and pelvic girdle Somites 26-32 Dorsal and intervertebral muscles Metameric level, specific origin In the heterotopic transplantations, the grafted somitic mesoderm gave rise to site-specific morphogenesis, irrespective of the cephalocaudal level of its origin. This result demonstrates that, at 2 days of incubation, the myogenic somitic cells are not regionalized.