Effects of inducers on inner and outer gastrula ectoderm layers of Xenopus laevis

Gastrula ectoderm, isolated from Xenopus laevis, was cultured in Holtfreter solution or modified Leibovitz medium (L-15) by the sandwich-method with or without inducer. The ectoderm (SD cell layers) consisted of two cell sheets, representing a superficial (S) and a deep (D) layer. In the L-15 medium rather than in Holtfreter solution, the two cell layers separate out into distinct cell masses. This difference in cell affinity under certain experimental conditions could indicate that the deep layer contains endodermal cells. However, an endodermal character of the deep layer can be ruled out by induction experiments with vegetalizing factor or dorsal blastopore lip as inducers. Under the influence of vegetalizing factor the outer as well as the inner ectoderm layer differentiated into mesodermal derivatives such as notochord and somites. The results of the experiments with dorsal blastopore lip as inducer indicate that both inner and outer ectoderm layers are responsive to the neural stimulus. The lower neural competence of the outer ectoderm layer observed by several authors in normogenesis is discussed with regard to the hypothesis about short distance diffusion of the neutralizing factor and/or close cell-to-cell contact between inducing tissue and ectodermal target cells.