ABSTRACT
An Artificial environmental stimulus of short duration (10-15 min.) leads to either mesodermal or neural differentiation of competent amphibian ectoderm in certain experimental conditions (Johnen, 1956a; Yamada, 1962). Subsequent to this stimulus development into well organized structures is autonomous, independent of environmental ‘inductive’ factors. Hence, two alternatives for the control mechanism have to be considered; either the whole pathway to the complex end-result is triggered and determined during the short-term initial pulse (primary induction), or only the first step of a long chain reaction has been activated, and this is followed by secondary interactions between the tissue components. Recently Grobstein (1963) discussed these alternatives in the mechanism of embryonic induction and concluded that a single step process is most unlikely in many known examples of induction (kidney tubule induction, induction of cartilage etc.). Seen from the viewpoint of primary induction, there seems to be good reason to agree with him, and we have in previous discussions often stressed the obvious significance of a neural/mesodermal interaction subsequent to the primary inductive stimulus (Saxén & Toivonen, 1961, 1962). The following experiment was planned to gain further evidence with this view in mind.
