Analysis of head and foot formation inHydra by means of an endogenous inhibitor.

In tissue regenerating the head, the ability to initiate head formation in a host increases with the time allowed for regeneration before grafting, while the foot-initiating ability decreases concomitantly. The reverse was found for tissue about to regenerate a foot. The early divergent changes thus indicated are counteracted in both head and foot regeneration by treatment with an inhibitor (Berking, 1977) in low concentrations.The inhibitor also interferes with processes which determine wether or not hypostome and tentacles are formed, and how many tentacles (if any) appear. The circumferential spacing of the tentacles was regular whether their number was normal or below normal.Secondary axes caused by implanted tissue either detach after having formed a head and a foot (i.e. behave like buds) or do not detach, having only formed a head. This alternative depends on the origin and amount of the implanted tissue and on the position of the implant within the host.The following model based on these findings is proposed: Head and foot formation start with pre-patterns which cause a continuously increasing change of the tissue's ability to initiate a head or a foot. Along the body axis this ability is determined by a graded distribution of "sources". As development progresses, the high source density which accumulates in the head region causes the formation of a hypostome and tentacles; the angular spacing of tentacles is also dependent on source density. At a certain low source density foot-formation is initiated. The inhibitor counteracts the increase of source density in head-forming tissue as well as the decrease of source density in foot-forming tissue. It thus appears to be part of the mechanism which controls morphogenesis in hydra.