The formation of the mesoderm in urodelean amphibians : III. The vegetalizing action of the Li ion.
Strong Li treatment leads to auto-activation of blastula and gastrula ectoderm due to lethal and sublethal cytolysis, resulting in adirect meso- and endodermization of the ectoderm. In contrast, a mild Li treatment-0.04 M LiCl in Holtfreter solution for only two hours-which does not show cytolizing effects, has no or only a weak mesodermizing effect upon blastula ectoderm. However, when the explant contains endoderm as well the same treatment leads to a marked transformation of ectoderm into mesoderm and subsequently into endoderm (vegetalization). Thisindirect action of the Li ion under "physiological" conditions apparently represents an enhancement of the normally occurring mesoderm formation from the ectodermal component of the blastula under the inducing action of the endodermal component and its subsequent transformation into endodermal structures, probably due to a rise in competence. The question is raised in how far the direct meso-and endodermization of gastrula ectoderm by heterogeneous inductors is open to the same criticsm as in the case of auto-activation of the ectoderm by a strong Li treatment. Finally, the experiments of Ave, Kawakami and Sameshima (1968) are briefly discussed.