ABSTRACT
The result of the activity of the primary organization centre in the amphibian gastrula is to determine a complex organized pattern of development. Within the ectoderm there arises a neural system in which, at the early stages, one can distinguish at least three different developmental types—brain, neural tube and neural crest ; another part of the animal hemisphere becomes mesoderm, and this can also be the result of an inductive action, since transplanted presumptive ectoderm can be caused to develop in this manner. There is general agreement that one cannot attribute these different end-results to any great extent to differences in the competence of the ectoderm submitted to the induction. There therefore remain two other factors to be investigated: differences either of a quantitative or qualitative nature within the organization centre, and differences in the type of contact between the various regions of the animal hemisphere and the organizer by which it becomes underlain. Waddington & Yao (1950) pointed out that there must be differences within the mesoderm in the rate at which evocating power is developed and in its final intensity, and they drew particular attention to the possible importance of the varying lengths of time during which the more anterior and the more posterior parts of the future neural system are in contact with inducing material.
