SummaryObservations in the point of union of the neuroectoderm with the nonneural ectoderm in chick embryos after treatment “in ovo” with cytochalasin B during neurulationTreatment of “in ovo” chick embryos with cytochalasin B during neurulation resulted in the failure of formation of the neural folds, producing an alteration in the point of union of the neuroectoderm with the nonneural ectoderm in its final development. This alteration consisted in the appearance of folding and superposition of both epithelia in this zone. The results support a possible role of the apical microfilaments in neuroectoderm and nonneural ectoderm in the process of neurulation.