A quantitative analysis of Triturus alpestris recombinates of newly invaginated "caudal" archenteron roof with either competent gastrula ectoderm or non-competent neurula ectoderm has demonstrated that neuralisation evoked by the underlying archenteron roof affects, in its turn, the regional differentiation of the latter. It does so by enhancing its notochordal differentiation; so, neural induction is clearly a two-way process. Somite differentiation is not significantly different in the two series. This is especially remarkable since both series show positive correlations between the amounts of notochordal and somite structures. The explantation is that the correlation coefficients differ considerably in the two series. The possible reason for this is discussed. Moreover the recombinates with competent gastrula ectoderm demonstrate that the size of the neural structures is determined by the total mass of the axial mesoderm and not by that of the notochord or that of the somites separately.