[Mechanically dependent heterotopias of the axial rudiments in clawed toad embryos]

Relaxation of tangential tensions was achieved in Xenopus laevis embryos at the stage of early gastrula by introducing radial cuts and insertion of ventral tissue wedges from the embryos at the similar stage. Velocity and direction of movement of the cellular material was compared in the normal and operated embryos using vital marking methods. We observed the formation of complexes of axial organs in atypical positions: after dorsomedial cuts, along the medial line and in the lateral blastopore lips; after anteroposterior (animal-vegetal) cuts, also on the ventral side; after lateral cuts (incisions), on the contralateral side of embryos. Ultrastructure of cells from the ventral region, which participate in the formation of neural plate, resembles that of the neural plate of the normal embryo as early as 4 h after surgery. We interpret these heterotopias of axial rudiments as the result of overtensions in relaxed tissues. The presence of overtensions was confirmed by tissue separation after local incisions. In all experimental series, axial rudiments were elongated in the direction of maximal overtension. Movements of the cellular material both in the normal and in the operated embryos were directed towards maximal tension.