Hydra transplantation phenomena and the mechanism of Hydra head regeneration. II. Properties of the head activation.

Measurements were made of the "head activation" in transplanted fragments of Hydra tissue. The measurements confirm that head activation is graded in the body and that activation increases during head regeneration. Experiments of two different sorts show that regeneration-specific activation increase is confined to the presumptive head zone. When head formation is initiated and subsequently blocked, the activation decays back to its original level in about 12 hr; this is three times faster than the activation decay observed in body tissue moved to a more basal position. Experiments on foot formation phenomena show a similar lability difference. It thus appears that regeneration-specific activation increase involves a mechanism different from the one responsible for the gradient of activation in the body. Cutting produces a local increase of the head and/or foot activation level; this increase decays in about 12 hr. These results and those of the preceding paper are consistent with a version of the Gierer-Meinhardt model which also accounts for the regulation of the head/body proportion in Hydra.