Types of Neural Tissues Induced through the Presumptive Notochord of Newt Embryo

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Neural induction through the presumptive notochord was tested by means of the sandwich method. The result disclosed that the notochord was a potent inducer of neural tissues not only in the ectoderm of gastrula but also in the ventral ectoderm of neurula and early tail-bud embryos. Structures formed by the induced neural tissue varied greatly. They can be classified into three types. (1) Tubular: the neural tissues induced in explants containing abundant mesenchymes always formed long tubular structures. The shapes of these neural tubes showed considerable variation; moreover, they were atypical and none formed the regular structure of the spinal cord. This type was most frequent, being found in about 50% of the explants. (2) Inverted: this type was produced when the explant contained mesenchymal component. Consequently, the epithelium of explants was missing. Nevertheless, a considerable mass of neural tissue was always induced. It was noticed that the induced neural tissues were invariably inside out; this type was found in about 30% of the explants.(3) Archencephalic: this was the only type to form the regular structure, i.e., the archencephalon. Formation of the archencephalon was limited solely to those explants containing only a few mesenchymes; this type was found in about 20% of the cases. As described above, it was found that the neural tissues induced by the same inducer of the notochord were not uniform but varied in type. Further it was shown that the types of neural tissue differed according to different quantities of the surrounding mesenchyme. Based on these facts, it is to be concluded that it is not the inducer of notochord, but the surrounding mesenchyme that is of primary importance for the determination of the types of neural tissue.