Precardiac mesoderm of chick embryos may be easily isolated from ecto-and endoderm with the aid of trypsin solutions. Cultivated alone in vitra it nevertheless differentiated into pulsating cardiac muscle tissue (Le Douarin et al., 1965a). In similar experiments with quail and mouse embryonie material, the differentation of precardiac mesoderm was also found to be independent of endoderm. In spite of this evident difference from amphibians, the presence of endoderm or at least other epithelial anlagen in cultures is necessary for the formation of tubular heart-like structures by explants of precardiac mesoderm.