Timing and mechanisms of mesodermal and neural determination revealed by secondary embryo formation in Cynops and Xenopus

We examined the timing and mechanisms of mesodermal and neural determination in Cynops , using the secondary embryo induced by transplantation of the prechordal endomesoderm. Two unique approaches were used: one was to observe gastrulation movements induced by the graft, and the other to measure the volumes of formed tissues. Transplanted graft pulled host animal cap cells inside to form a new notochord and other mesoderm of the secondary embryo, showing determination of mesoderm during gastrulation. The graft attained a certain width beneath the host ectoderm and moved near to the animal pole of the host by late gastrula, and a neural plate, which had a similar width to the graft, was formed covering the graft. The volume of neural tissues of the secondary embryo at tail-bud stages was about half that of the normal embryo, while the volumes of notochord were comparable in each case. These data suggest that prechordal endomesoderm, rather than notochord, determines the limit of neural plate in the overlying ectoderm. Similar dorsal grafts were transplanted at early gastrula in Xenopus but did not form well developed secondary embryos, demonstrating that the timing and mechanisms of mesoderm formation in Xenopus are different from those in Cynops .