VERTICAL VERSUS PLANAR INDUCTION IN AMPHIBIAN EARLY DEVELOPMENT

In the Urodeles, the archenteron roof invaginates as a single continuo us sheet of cells, vertically inducing the neural anlage in the overly ing ectoderm during invagination. The induction comprises first the ac tivation process, leading to forebrain differentiation tendencies, and then the superimposed transformation process, which changes presumpti ve forebrain development into that of hindbrain and spinal cord acting with a caudally increasing intensity. The activating action, being ma ximal anteriorly, decreases caudally to nearly zero. In the double-lay ered 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 al ready initiating vertical neural induction in overlying ectoderm at st ages 10 to 10(+) before any visible archenteron invagination. In Xenop us incomplete exogastrulae the prechordal mesoderm involutes normally prior to evagination of the endoderm and mesodem. Artificially produce d Xenopus total exogastrulae, made at stage 9 before mesoderm involuti on, behave just like axolotl total exogastrulae, showing no neural dif ferentiation. The notion of planar neural induction in Xenopus can onl y be applied in exogastrulae and Keller explants for the transforming action, which is maximal in the caudal archenteron roof. In normal Xen opus development, the formation of the entire nervous system is essent ially due to vertical induction by the successively involuting prechor dal and notochordal mesoderm. The different behavior of Xenopus embryo s in comparison with Urodele embryos can essentially be explained by t he double-layered character of the animal moiety of the Xenopus embryo .