The tentacles in hydra have characteristics of both spacing patterns and number-regulating patterns in that their number under some circumstances changes with the size of the animal and under others does not. To determine which type of processes could yield these results, an extensive analysis was undertaken of the size parameters pertinent to tentacle formation. To do this pieces of tissue, varying in shape and spanning a 30-fold size range, were excised and allowed to regenerate into complete animals. Three separate mechanisms were found to combine which resulted in the final number of tentacles: (1) the spread of the two-part head pattern to produce a competent band of tissue of a given size where the tentacles could form; (2) initiation of tentacle-forming centres by a spacing mechanism and (3) growth of the tentacles to a size proportional to the size of the animal.

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