The Concept of Embryonic Induction in the Passage of Time

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The paper critically reviews, in historical perspective, the theoretical aspects of interactions of germ layers, which control subsequent differentiation of somatic tissues in amphibian embryos. The adequacy of the classical argument which resolved the organizer action into actions of individual inducing substances is questioned, and the usefulness of the theoretical models which operate with morphogenetic events as opposed to inducing substances is emphasized. It is pointed out that the prospective ectoderm of an early amphibian gastrula has the ability to produce almost all somatic tissues belonging to the three germ layers under various epigenetic conditions, and hence omnipotency could be ascribed to it. The importance of the rather recent recognition that the developmental pathways of the mesoderm are established under the influence of the endoderm, before organizer action starts, is stressed. The influence of the endoderm is interpreted as vegetalization of the animal hemisphere. A review of the studies on some heterogenous inductors, which were originally intended to characterize the chemical factors involved in organizer action, shows that the main components of those inductors have strong vegetalizing effects on competent ectoderm. Thus the effect of those inductors and their effective molecules should be evaluated for the mechanism of vegetalization of the upper hemisphere before gastrulation, rather than for the mechanism of the organizer action itself. The regionalization of neural tissue produced by heterogenous vegetalizing inductors could be due to a secondary interaction between the ectoderm and mesoderm which has been produced by the vegetalizing effects of those inductors. If this is the case, those inductors do not give any clue as to the chemical factors involved in regionalization by the organizer in normogenesis, in contradiction to the past dominating attitude in this area. The reason that one should take into consideration a possible causal role of morphogenetic movements in the mechanism of regionalization is presented. My task is to critically review the studies of developmental interactions between germ layers of amphibian embryos leading to the appearance of somatic tissues, an area to which Pieter Nieuwkoop contributed a great deal. The focus of the paper is on the theoretical aspects of the problem as they evolved in the last three decades, and not on the experimental data, most of which have been reviewed rather recently (NAKAMURA, 1978; HAMA, 1978; KAWAKAMI & SASAKI, 1978; NIEUWKOOP, 1973; TAKAYA, 1978; TIEDEMANN, 1978; TOIVONEN, 1978). Unless otherwise indicated, the experiments mentioned in this paper concern Urodele embryos.