Positional information and pattern regulation in Hydra. Dynamics of regions at the boundary.

The dynamics of boundary regions have been investigated mainly by axial grafting and with the emphasis on the head end. The time to resist inhibition of head-end formation and the ability to inhibit head-end formation have been assayed under a variety of conditions. The times increase with distance from the head end. The times required by a boundary region to aquire the inhibitory properties of a head end are longer than those required to aquire resistance to inhibition. Determination of a head end is faster at a cut surbface and at higher temperatures. The results are discussed in terms of a model involving two gradients. Some anomalous results are reported.