Bipolar Inhibitory Gradients' Influence on the Budding Region ofHydra viridis

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A head inhibitory gradient arising in the vicinity of a hydra's head, and a foot inhibitory gradient arising from the animal's foot are thought to intersect in the budding region. To investigate the possible influences of these gradients on budding, samples were prepared of 25 or more animals having as many as five gastric regions or gastric-plus-budding regions grafted in tandem. The segments of grafted animals were considered equivalent except for distance from the head and foot. The average frequencies of regeneration of budding regions on grafted gastric regions, and the average number of buds on grafted budding regions were determined for each sample. Both of these variables changed as functions of distance from the head in U-shaped curves as the hypothesis predicts. The arm of the U closest to the head indicates a stronger degree of inhibition than the arm closest to the foot since there are less regeneration and fewer buds close to the head. Evidence for segments on which ectopic heads regenerated also shows inhibition of budding due to a head. The slopes of the arms of the U-shaped curves do not resemble the slopes of the curves for head and for foot inhibition, however, particularly since the slopes for budding inhibition do not decrease as a function of the number of grafted segments. It is doubtful, therefore, that the observed bipolar inhibition of the budding region is due to the inhibitors of head and foot formation.