The budding region as source of diffusible inhibitors of head and foot regeneration inHydra viridis

An implication of Crick's (1970, 1971) "source-sink" model of diffusion gradients is that the addition of sources along the length of a gradient would cause it to bulge away from linearity in the direction of the source. The activity of the budding region ofHydra viridis as a source of the diffusible inhibitors of head and of foot regeneration is investigated in this light, by placing multiple series of budding regions along the length of the inhibitory gradients of head and of foot regeneration previously described (Shostak, 1972, 1973).Samples of 30-50 animals were used to determine the frequencies of head and of foot regeneration at each graft border formed by grafting 2 to 5 gastric-plus budding regions in tandem, the distal one having a terminal apical head, and the proximal one part of a host animal having terminal basal peduncle and foot. These frequencies were compared to the corresponding frequencies at appropriate distances along the lengths of the inhibitory gradients, as computed from the linear equations for these gradients based on earlier work. The curves for the regeneration of heads and feet for animals with three or fewer additional budding regions deviate in the direction of greater inhibition, but do not differ significantly from the gradients on animals having only grafted gastric regions. The curves for animals with four additional budding regions, however, bulge out toward greater inhibition, and differ with statistical significance from the linear inhibitory gradients at the fourth graft border.The results show, therefore, that the budding region is a source of both the inhibitors of head and of foot regeneration also produced by the head and foot, respectively. The suggestion arises that the diffusible inhibitors produced by the morphogenetically active regions ofHydra have no effect on the normal homeostatic processes and budding occurring in these regions, but normally prevent regeneration that might otherwise occur during cellular turnover.