UsingXenopus blastulae of stage 9-, recombinates were made of the animal, ectodermal cap (zones I.II) and the vegetative, endodermal yolk mass (zone IV) (see Fig. 1). For the experiments either the entire ectodermal cap (A.B), the single outer layer (A) or the stratified inner layer (B) were used.A comparison of the quantitative composition of the recombinates and the corresponding isolates-on the basis of absolute values expressed in units of section surface area-demonstrates unequivocally that the entire mesoderm originates from the ectodermal "half" of the anuran egg under an inductive influence emanating from the endodermal "half". This holds for recombinates of the vegetative yolk mass with the entire ectodermal cap as well as with its outer or inner layer alone.A comparison of mesoderm formation in recombinates of the entire ectodermal cap or with its outer or inner layer with the vegetative yolk mass shows that in all cases mesoderm formation is proportional to the amount of ectoderm available. In addition, the outer layer of the ectoderm is partially endodermized which may be brought in relation with the fact that in normal development an endodermal lining extends upwards from the endodermal mass, which, among other things, covers the prechordal mesoderm on the outside.The outer layer of the ectoderm has markedly lower neural competence than the inner layer, from which in normal development the bulk of the neural material arises.