Ultrastructural changes occur in the nuclei of the presumptive ectoderm cells of theearly gastrula of Cynops orientalis after treatment with a mesodermal inducing substance(guinea pig bone marrow extract). The large condensed chromatin masses in the nuclei ofthe control series are no longer found; they become more or less dispersed. The successivechanges of chromatin from condensed to dispersed have been observed. And in connectionwith the observations on neural induction after Ca-free treatment, the transformation fromheterochromatin to euchromatin as the initiation of cell differentiation has been discussed. Ultrastructural changes occur in the nuclei of the presumptive ectoderm cells of the early gastrula of Cynops orientalis after treatment with a mesodermal inducing substance (guinea pig bone marrow extract). The large condensed chromatin masses in the nuclei of the control series are no longer found; they become And in connection with the observations on neural induction after Ca-free treatment, the transformation from hepochromatin to euchromatin as the initiation of cell differentiation has been discussed.