Head formation at the basal end and mirror-image pattern duplication in Hydra vulgaris.

Head and foot in Hydra are organizing centers and considered to be sources of long-range inhibitory morphogens that prevent head and foot formation elsewhere. In a previous study the apparent long-range head inhibition was shown to coincide with long-range foot promotion exerted by the head. Here it is shown that: (1) ring-shaped pieces of the body column taken from a near-foot position form feet - frequently circular - if inserted into the midgastric region; this ectopic foot formation is strongly dependent on assistance by the head. (2) Bisection causes a transient increase in positional value at the wounded basal end of the upper body column. This transient development in the head direction in turn promotes ectopic foot formation by transplants and thus has an effect as though the source of a foot-inhibiting morphogen were removed. The existence of long-range foot inhibition is open to question. (3) If a ring with low positional value is present in the midgastric region, the increase in positional value at the basal end is stable and results in mirror-image head formation instead of foot regeneration in up to 100% of cases. Even before the ring forms a foot it acts like a ligature and subdivides the body column into two developmental compartments. (4) The basal head in turn organizes a mirror-image duplication of the body pattern. In pattern regulation, Hydra follows rules of intercalation known from other organisms.