ABSTRACT
The active principle (the evocator) of the amphibian organisation centre is very widely distributed throughout the animal kingdom. We owe our knowledge of its occurrence mainly to the investigations of Holtfreter (1934), who systematically tested the inducing properties of tissues belonging to most of the main phyla, by the well-known technique of implanting them into the blastocoele of young newt gastrulae. Some of the tissues were poisonous to the gastrulae when they were implanted in the living state, but many of them could be rendered innocuous by a short immersion in boiling water. This treatment seemed in general to have little effect on the inducing properties of tissues which were non-poisonous in the living state, and it seems probable that the effect of the boiling on poisonous tissue has been to annul the deleterious effects of the implants rather than to alter their inducing properties. However, Holtfreter (1933) has also shown that in the ventral ectoderm and in the endoderm of the newt gastrula, the evocator exists in a masked form, so that these tissues have no inducing power until they have been killed ; and we cannot be certain that a similar state of affairs is not found in those tissues which are too poisonous to be tested while still alive.
