This problems disappears when the spacing between the head and foot system is achieved by an interaction via the source density. As mentioned above, the head activation appears at the position of the highest source density. Nevertheless, head and foot system can coexist at a close neighbourhood in small animals since no direct inhibition is involved. The graded source density only generates a preference. Experimental evidence indicates that the foot also lowers the source density, contributing in this way to the maintenance of the source density gradient.
Many structures emerge during development close to each other in a precise arrangement.
In Hydra , the tentacles appear around the hypostome, the opening of the gastric column. Many experiments can be accounted for by the assumption that tentacles are under control of a separate activator-inhibitor system that also depends on the source density. Since the source density increases under the influence of the primary head system, the latter generates the precondition for tentacle initiation. Locally, however, the head signal suppresses tentacle formation. The model accounts for a strange-appearing observation.