The still unsegemented axial mesoderm was cross cut at different distances form the last somite under the normal physiological conditions, at the low temperature and under the effect of cytochalasin B. Under the normal conditions the inner cells of the axial mesoderm formed a lens-shaped cleft immediately after the operation; this movement was suppressed at the low temperature and absent under the effect of cytochalasin B. This suggests the active contractility of the mesodermal syncytium. The wound margin near the last somite acquired the epithelial structure already within several minutes after the cut, whereas in the more caudal regions it preserved the connective tissue character for a long time. The importance of the axial mesoderm contractility and its ability of epithelization for the somitogenesis is discussed.