[Mitotic activity of the embryonic tissues of the clawed toad in the period of gastrulation and neurulation].

The mitotic indices (MI) were measured on serial transverse sections in ecto- and mesoderm of different dorsoventral zones of the X. laevis embryos from the late blastula stage till the end of neurulation. The MI were shown to fall simultaneously in mesoderm of all zones during the transition from middle to late gastrula (stages 12 and 13) and in ectoderm of all zones at the late gastrula stage (12 1/2 and 13) and rise synchronously in the dorsal and lateral ectoderm and lateral mesoderm during neurulation. In the most cases the rise of MI coincided with the periods of active morphogenetic movements of the given area and the fall of MI with the cessation of these movements.