ABSTRACT
As shown by Holtfreter (1944, 1945, 1947), the isolated presumptive ectoderm of Triturus torosus develops into a mass of epidermal cells when cultured in a physiological medium. However, if it is treated with injurious media such as acid, alkaline, or Ca-free solutions, or alcohol, and then cultured in a physiological medium, it differentiates neural structures and mesenchyme. These observations were confirmed in experiments made with the ectoderm of T. pyrrhogaster (Yamada, 1950; Karasaki, 1957a) except for the following points: treatment with Ca-free media or with alcohol does not cause neural or mesenchymal differentiation in the ectoderm of this species, although these treatments lead to a disaggregation of the ectoderm just as with acid and alkali (Karasaki, 1957a). For the ectoderm of T. pyrrhogaster ammonia was found most effective in producing neural structures. Even differentiation of the mesoderm is altered by treating with ammonia: an isolate of the ventral marginal zone of T. pyrrhogaster, with an inherent tendency to form blood-island and mesothelium, differentiate notochord and muscle after a treatment with ammonia (Yamada, 1950). It was emphasized by Yamada that ammonia shifts the mode of differentiation of the isolated ectoderm and mesoderm from ventral towards dorsal. Such effects were designated as dorsalization. Similarly it was demonstrated by Kawakami & Okano (1955) that lactic acid is capable of dorsalizing the presumptive ectoderm and ventral marginal zone.
