In the previous papers it was shown that three factors are involved in tne regulation of hydra into a two-compartment pattern comprising hypostome and non-hypostomal regions (Webster & Wolpert, 1966a; Webster, 1966). These three factors are time for hypostome determination, level of inhibition, and threshold for inhibition. All these factors appear to be graded along the axis. The interaction between the three factors was shown to be capable of accounting for polarized regulation, but it was pointed out that a more detailed explanation would necessitate knowledge of the dynamics of these factors during regulation. In particular it is important to know how threshold and level of inhibition change during regulation, and how the gradients themselves are controlled. Such data is also required to enable an explanation for a variety of grafting experiments to be offered.
The hydra used throughout this work was Hydra littoralis. Full details of experimental methods have been given in Webster & Wolpert (1966a).