The first attempts to produce a capacity for induction in tissue which is normally incapable of performing such an action were made by Spemann and Geinitz in 1927. They grafted a fragment of presumptive ectoderm into the organization centre of another embryo, and, removing it a few hours later, found that it had been “infected” with the inducing capacity of the tissues by which it had been surrounded. The experiment inevitably suggested that the inducing capacity is the property of a chemical substance which had diffused out of the organizer tissue into the grafted ectoderm fragment. A similar hypothesis could be used to explain the observation of Mangold and Spemann (1927) that in normal development the presumptive neural plate acquires inducing capacity at the same time and in proportion as it is underlain and determined by the mesodermal organizer. The first suggestion that the non-inducing parts of a Urodele gastrula themselves possess an organizing capacity, which is masked but only awaits activation or release, emerged in the work of Dürken (1926), Bautzmann (1929,
a
,
b
), Kusche (1929), and Holtfreter (1931), and attention was first drawn to it by Huxley (1930). The German authors showed that if fragments of the gastrula are “interplanted” into the body cavity or optic vesicle of older larvae, they may develop into something other than their presumptive fate, and in particular, presumptive epidermis or neural plate may develop into various mesodermal derivatives such as notochord or muscle. Huxley pointed out the similarity between this phenomenon, which was called
bedeutungsfremde Selbstdifferenzierung
, and the results of isolating parts of the axial gradient system of lower organisms, which have been particularly described by Child (summaries 1928, 1929). An isolated part of an axial gradient system reconstructs a “dominant region”; and Huxley suggested that we could account for
bedeutungsfremde Selbstdifferenzierung
by supposing that an isolated part of a gastrula reconstructs the dominant region,
i.e
., the organization centre. In the spring of 1932 one of us (C. H. W.), while on a visit to the laboratory of Dr. O. Mangold in Berlin for the purpose of learning the technique of amphibian operations, attempted to carry the matter a step further. If Huxley’s explanation were correct, one would have to suppose that a capacity for behaving like a “dominant region”, that is, for inducing, is latent in the presumptive ectoderm, and this capacity should become manifest when the ectoderm changes into a dominant region after isolation. The following experiment was therefore made to test this point. Fragments of presumptive ectoderm from a young gastrula were interplanted into the eye-cavity of Anuran tadpoles, from which the eye-ball had previously been removed. After two days the interplanted tissue was removed and grafted by the
Einsteck
method into the blastocoele of young newt gastrulae, to discover whether they were capable of inducing the formation of neural plate. Three sets of controls were made. In one set organizing tissues were interplanted for two days and then tested to see whether their inducing capacity had been impaired, in the second set organizing tissue was isolated for two days in Holtfreter solution, and then tested, and in the third set presumptive ectoderm was isolated for two days in Holtfreter solution and tested for inducing capacity.
Studies on the nature of the amphibian organization centre III—The activation of the evocator