The nuclear specificity of 12 day embryonic skeletal muscle cells has been studied in ectopic limbs that developed after implantation of quail limb somatopleural mesoderm into chick hosts. In eleven cases, the nuclearity of the myotubes was homospecifically of chick type. In the remnant eight cases, the musculature was heterospecific, the myotubes being homospecific and constituted by chick or quail nuclei or the myotubes being heterospecific and containing at random chick and quail nuclei. The heterospecific multinuclearity was analyzed in 85 portions of myotubes. The degeneration of numerous heterospecific muscle cells was attested by the shrinked and hyperchromatic states of the nuclei and by the fragmentation of the sarcoplasm that contained cellular debris. Nevertheless, there is no correlation between myotube degeneration and heterospecificity, because the same necrotic figures are observed in chick host homospecific myotubes that have reached the same degree of development.