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

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Abstract
Mesoderm formation in the presumptive trunk organizer was analyzed in gastrulae of C. pyrrhogaster. The presumptive trunk organizer showed little or no mesodermal differentiation in the beginning gastrula (0 h embryo). When the presumptive trunk organizer came into contact with the newly invaginated cranial archenteron roof, it rapidly formed mesoderm. This differentiation may be brought about by an inductive effect of the underlying cranial archenteron roof. To investigate 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. The prechordal plate, which came into contact with the still uninvaginated presumptive trunk organizer, stimulated dorsalization of the weakly mesodermized trunk organizer. The stepwise process of mesoderm formation in the trunk organizer is discussed.

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