Change of the differentiation pattern of amphibian ectoderm after the increase of the initial cell mass.

Early amphibian gastrula ectoderm (Triturus alpestris) has been treated with vegetalizing factor. While normal sandwiches (animal caps of two eggs) differentiated mainly into endoderm derived tissues, giant-sandwiches (a combination of 8 animal caps) formed mesodermal and neural tissues in addition. The results support the interpretation that ectoderm will differentiate into endoderm derived tissues when all or nearly all cells are induced (presumably depending on certain threshold concentrations of the inducer). This is the case in the normal sandwich after treatment with high concentrations of vegetalizing factor for 24 h. However, in a giantsandwich it must be assumed that only the cells in the vicinity of the inducer will be triggered to differentiate into endoderm derived tissues. Mesodermal structures will be formed by secondary interactions between the induced ectoderm (endoderm) and non induced ectodermal cells. The induction of neural structures could be explained as a further interaction between mesodermalized and non induced ectodermal cells. This chain of events is compared with the steps of determination in normogenesis.