Neural crest replaced by gastrula ectoderm in amphibia. Effect on neurulation, CNS, gills and limbs.

Early Axolotl gastrula ectoderm was grafted into early Triturus neural stages in place of excised neural folds at the gill and anterior trunk level. Macroscopically the young graft behaves like normal neural fold material: it follows the closing host neural plate to the dorsal midline, folds into the host's interior and, especially in the gill region, moves ventrad beneath the host's epidermis. These movements cannot be interpreted as active migration. They are the result of passive displacements by morphogenetic forces inside the embryo. Histologically the graft differentiates into neural and neural crest tissue, the quantitative relation depending on the host's region. At the gill level the graft forms mesenchyme and other neural crest elements and hardly any neural structures. In the trunk about one half of the graft forms a secondary, surplus CNS. Problems of induction, differences between gill and trunk region and between graft and normal fold behaviour are discussed. Limbs develop normally. The dorsal layer of the blastema is furnished by graft cells. Host and graft tissue can stay separate or form a combined blastema.