Establishment of the organizing activity of the lower endodermal half of the dorsal marginal zone is a primary and necessary event for dorsal axis formation in Cynops pyrrhogaster.
The isolated upper marginal zone from the initial stage of Cynops gastrulation is not yet determined to form the dorsal axis mesoderm: notochord and muscle. In this experiment, we will indicate where the dorsal mesoderm-inducing activity is localized in the very early gastrula, and what is an important event for specification of the dorsal axis mesoderm during gastrulation. Recombination experiments showed that dorsal mesoderm-inducing activity was localized definitively in the endodermal epithelium (EE) of the lower marginal zone, with a dorso-ventral gradient; and the EE itself differentiated into endodermal tissues, mainly pharyngeal endoderm. Nevertheless, when dorsal EE alone was transplanted into the ventral region, a secondary axis with dorsal mesoderm was barely formed. However, when dorsal EE was transplanted with the bottle cells which by themselves were incapable of mesoderm induction, a second axis with well-developed dorsal mesoderm was observed. When the animal half with the lower marginal zone was rotated 180° and recombined with the vegetal half, most of the rotated embryos formed only one dorsal axis at the primary blastopore side. The present results suggest that there are at least two essential processes in dorsal axis formation: mesoderm induction of the upper marginal zone by endodermal epithelium of the lower marginal zone, and dorsalization of the upper dorsal marginal zone evoked during involution.