An essay on the similarities and differences between inductive interactions in anuran and urodele embryos

As a first step towards providing a conceptual approach to understanding similarities and differences in the mechanisms which guide inductive interactions among related organisms (e.g. various amphibia), a set of five principles is offered here. These principles were formulated by analyzing literature examples of classical embryological phenomena and by performing experiments with activin, a peptide growth factor which is currently suspected to play for a role in mesoderm induction. Mechanisms which account, at least in part, for the observed differences between anuran and urodele inductive processes can be derived from these principles.