Vertical versus planar induction in amphibian early development.

In the Urodeles, the archenteron roof invaginates as a single continuous sheet of cells, vertically inducing the neural anlage in the overlying ectoderm during invagination. The induction comprises first the activation process, leading, to forebrain differentiation tendencies, and then the superimposed transformation process, which changes presumptive forebrain development into that of hindbrain and spinal cord acting with a caudally increasing intensity. The activating action, being maximal anteriorly, decreases caudally to nearly zero. In the double-layered Xenopus embryo, the internal mesodermal marginal zone shows much more independent and earlier regional segregation and involution than the external marginal zone in the Urodeles; its prechordal mesoderm already initiating vertical neural induction in overlying ectoderm at stages 10 to 10 before any visible archenteron invagination. In Xenopus incomplete exogastrulae the prechordal mesoderm involutes normally prior to evagination of the endoderm and mesodem. Artificially produced Xenopus total exogastrulae, made at stage 9 before mesoderm involution, behave just like axolotl total exogastrulae, showing no neural differentiation. The notion of planar neural induction in Xenopus can only be applied in exogastrulae and Keller explants for the transforming action, which is maximal in the caudal archenteron roof. In normal Xenopus development, the formation of the entire nervous system is essentially due to vertical induction by the successively involuting prechordal and notochordal mesoderm. The different behavior of Xenopus embryos in comparison with Urodele embryos can essentially be explained by the double-layered character of the animal moiety of the Xenopus embryo.