Studies on the formation and state of determination of the trunk organizer in the newt Cynops pyrrhogaster. II. Inductive effect from the underlying cranial archenteron roof

Mesoderm formation in the presumptive trunk organizer was analyzed in gastrulae of Cynops pyrrhogaster. The presumptive trunk organizer showed little or no mesodermal differentiation in the beginning gastrula (0 h embryo). But as soon as the presumptive trunk organizer came into contact with the newly invaginated cranial archenteron roof, it rapidly formed mesoderm. This suggests that this differentiation was brought about by an inductive effect of the underlying cranial archenteron roof. For investigation of this possibility, the presumptive trunk organizer of 0 h embryos (Tr-0) and the newly invaginated cranial archenteron roof (presumptive pharyngeal endoderm and prechordal plate) of successive stages were cultured in isolation and by the sandwich technique. The newly invaginated presumptive pharyngeal endoderm and prechordal plate had no effect on mesoderm formation of the presumptive trunk organizer, and mesodermal differentiation of the combinations was similar to that of the Tr-0 alone. On the other hand, results showed that the prechordal plate, which came into contact with the still uninvaginated presumptive trunk organizer, stimulated dorsalisation of the weakly mesodermized trunk organizer. Based on these results, the stepwise process of mesoderm formation in the trunk organizer is discussed.