Regulation in hydra is polarized, the new hypostome always forming at the distal end of an isolated piece. Polarity is rigorously maintained in such pieces but can be altered in graft combination (Peebles, 1900; King, 1901; Morgan, 1901; Browne, 1909; Goetsch, 1929; Tardent, 1960; Webster, 1966b).

Previous investigations (Webster, 1966a, b; Webster & Wolpert, 1966) have suggested that polarized regulation occurs as a result of the interaction of three factors: time for hypostome determination, inhibition of hypostome determination and threshold for inhibition. All these factors appear to be graded along the axis. There is some evidence that time for hypostome determination and threshold for inhibition are closely linked factors (Webster, 1966b) and in many experiments it is not possible to distinguish between them. For this reason they are referred to throughout this paper as ‘time-threshold’ factors or properties.

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