Studies on the formation and state of determination of the trunk organizer in the newt,Cynops pyrrhogaster : IV. The association of the neural-inducing activity with the mesodermization of the trunk organizer.

The stepwise process of the formation and determination of mesoderm inCynops pyrrhogaster was analyzed. The presumptive ectoderm (PE) of the early gastrula was transformed into mesoderm within 12 h when transplanted into the upper half of the dorsal marginal zone of the same stage. The self-differentiation capacity and the neural-inducing activity of this newly mesodermized PE (MPE) were examined by both isolation and sandwich cultures.The MPE showed self-differentiation for notochord and muscle in the isolation culture. In the sandwich culture, the MPE made contact with the PE of the successive gastrula stages. The MPE was capable of inducing neural tissues even in the PE of the mid-gastrula, which has high neural competence but loses it within a short period of 6 h.These results show that firstly the mesodermization of the PE is completed within 12 h and secondly both the self-differentiation capacity and the neural-inducing activity are established immediately after the mesodermization of the PE.