[The influence of Li- and SCN-ions on the differentiation capacities ofAmbystoma-ectoderm and their changes as a result of combined treatment with both ions].

1. Under the influence of NaSCN-solution (experimental series NaSCN) explants of competent ectoderm from the early gastrula stage ofAmbystoma mexicanum differentiate into archencephalic structures. Treatment with LiCl-solution (series LiCl) evokes mesodermal and endodermal differentiation tendencies, as well as spinocaudal tendencies which are probably of a secondary nature. 2. Ectoderm which is treated first with LiCl-solution and then with SCN-ions (series LiCl/NaSCN) shows an archencephalic induction effect, which is much more pronounced than in the NaSCN series without pretreatment with Li-ions. 3. In reciprocal experiments (series NaSCN/LiCl) the reaction system forms, besides some mesodermal organs, mainly endodermal structures. They are realized in much higher frequency than in the LiCl experiments without pretreatment with NaSCN-ions. 4. Neural crest derivatives are present in varying frequencies in all series. 5. Prom these results the following conclusions can be drawn: SCN- and Li-ions have an opposite influence on the differentiation capacities of competent ectoderm. Successive treatment with both ions leads to the suppression of those differentiation tendencies that are stimulated by the first inductive influence. The result is an induction effect which is characteristic for the induction factor that acts last. 6. The ectoderm is activated by pretreatment both with SCN- and Li-ions, so that the succeeding endodermizing influence of Li-ions and the archencephalizing effect of SCN-ions is more pronounced than in non-pretreated ectoderm.