Pattern formation in early developmental stages of amphibian embryos.

An internal factor (or factors), which determine the endoderm and the mesoderm anlage, seem to exist already in certain regions of the oocyte and very early developmental stages. The factor(s), which is (are) protein in nature, has (have) similar properties to the one isolated from chicken embryos. The induction of the neural plate seems to be mediated by a membrane mechanism. The segregation of the primarily formed anlagen occurs by secondary tissue interactions.