Neural Activation and Transformation in Explants of Competent Ectoderm under the Influence of Fragments of Anterior Notochord in Urodeles

Abstract

ABSTRACT The inductive influence of presumptive notochordal tissue (neurula stage) upon competent ectoderm was tested in sandwich-experiments, for which eggs of Triturus alpestris, T. taeniatus, and Ambystoma mexicanum were used. Strong activating and rather weak transforming influences emanate from the anterior portion of the notochord. Both activating and transforming influences have the character of field actions and spread with decrement, probably from cell to cell, in the overlying ectoderm. Both influences, but especially the activation, can spread over an area much larger than the area in which a direct contact between inductor and ectoderm occurs. It is therefore suggested that the more lateral portions of the archenteron roof merely support the activation spread from the dorsal midline, probably as a form of double assurance. The activation leads to a segregation into neural, mesectodermal, activated epidermal, and indifferent epidermal zones, each of which is characterized by a certain threshold value. In the activated epidermal zone differentiation tendencies for prosencephalic placodal development arise. They can be transformed by additional transforming influences into those for rhombencephalic or spinal placodes.