Experiments are described in which in early to late blastulae ofAmbystoma mexicanum (stages 7-8/9 Harrison) the animal, ectodermal "half" (zones I.II) was combined with the vegetative, endodermal yolk mass (zone IV) in various orientations, viz. in random orientation or with the dorso-ventral axes of the two components in identical, opposite or perpendicular orientation (0°, 180°, or 90° translocation respectively). The results demonstrate unequivocally that the dorso-ventral polarity of the induced mesoderm, and thus of the embryo, depends exclusively upon the inherent dorso-ventral polarity of the endoderm, whereas the grey crescent, a considerable part of which is located in the animal, ectodermal "half', plays no causal role whatsoever.The results also show that the dorso-ventral polarity is inherent in the entire endodermal mass, but that the subsequent regional differentiation of the endoderm depends upon stimulating influences emanating from the surrounding mesoderm, the later nutritive yolk representing that part of the endoderm which normally does not come under the influence of the mesoderm, and therefore fails to receive the necessary stimulus for further differentiation.On the basis of these findings "Schultze's Umkehrexperiment" as studied byPENNERS andSCHLEIP, PENNERS, andPASTEELS are reinterpreted, whileDALCQ andPASTEELS' general developmental theory as well asCURTIS' cortical grafting experiments are critically discussed.