On the basis of certain earlier suggestions made by Lehmann (1950) and Yamada (1950), together with our own experimental data, a modification of the two-gradient hypothesis of primary induction was presented by us some years ago (Toivonen & Saxén, 1955). Subsequently, this theory has often been referred to, accepted or criticized, and even misunderstood. There may thus be reasons for discussing it in the light of some recent experimental data.
At present there are limits to our opportunities of studying what is obviously the most important point in embryonic induction, the induction process itself. Simultaneously with such experiments on the induction process it is therefore necessary to continue research work on classical lines, and to obtain a further clarification of the causal relationships between the inductor and its morphogenetic action. A variety of qualitative investigations in this category have been made, but conceptions of the different quantities and the ratios of the active agents which participate in the primary induction are still based on indirect data.
Stem Cell Terminology and ‘Synthetic' Embryos: A New Debate on Totipotency, Omnipotency, and Pluripotency and How It Relates to Recent Experimental Data
