Evidence of morphogenetically significant diffusion gradients in Hydra viridis lengthened by grafting.

Morphogenetic inhibitory gradients were produced in Hydra viridis having 2-5 gastric regions or 2-5 peduncles grafted in tandem. The frequency of head regeneration at graft borders changed as a function of distance from the terminal head, and the frequency of foot regeneration changed as a function of distance from the terminal foot. The object of the study was to characterize and compare these morphogenetic inhibitory gradients in light of Crick's (1970) model of diffusion gradients. Heads regenerate more easily on gastric regions and feet on peduncles. This is shown in several ways: (1) on gastric regions more heads and fewer isolated tentacles and 'spikes' (isolated hypostomes) form, while the opposite is true on peduncles; (2) more heads are regenerated at graft borders between gastric regions than at those between peduncles at comparable distances from the terminal head, while more feet are regenerated at graft borders between peduncles than at those between gastric regions at comparable distances from the terminal foot; (3) the Y intercepts for gradients of the frequencies of head and of foot regeneration as functions of distance (which are the theoretical frequencies of regeneration at the ends of the animal) are higher for head regeneration on gastric regions than on peduncles, and for foot regeneration on peduncles than on gastric regions. This is explained in terms of relative differences in either competence to regenerate or sensitivity to inhibitors. The slopes of the frequencies of head and of foot regeneration as functions of distance from the respective ends change inversely with the numbers of gastric regions and peduncles grafted. This is explained in terms of changes in the distance between the sources of diffusible inhibitors and the sinks which use them up. The results are taken to support Crick's model in which a linear gradient of a diffusible substance is produced between its source and sink.