Inductive interactions in early amphibian development and their general nature.

After a short discussion on cell interactions in general and inductive interactions in particular, the almost completely epigenetic nature of amphibian development is emphasized. In the symmetrized egg undergoing cleavage a large-scale inductive interaction occurs which leads to the formation of the meso-endoderm. Meso-endoderm formation gives rise to the morphogenic process of gastrulation. In the ensuing triple-layered embryo inductive interactions are strongly enhanced. The following large-scale inductive interaction leads to the formation of the neural anlage. This is again followed by the morphogenetic process of neurulation or neural tube formation. Subsequent interactions between the germ layers of the triple-layered embryo give rise to the formation of the regional pattern of organ anlagen. Finally, the most promising approaches to the nature of inductive interactions for mesoderm and endoderm formation are discussed.