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 sou rces of long-range inhibitory morphogens that prevent head and foot fo rmation elsewhere. In a previous study the apparent long-range head in hibition 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 circula r - if inserted into the midgastric region; this ectopic foot formatio n is strongly dependent on assistance by the head. (2) Bisection cause s a transient increase in positional value at the wounded basal end of the upper body column. This transient development in the head directi on in turn promotes ectopic foot formation by transplants and thus has an effect as though the source of a foot-inhibiting morphogen were re moved. 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 a nd 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 lik e a ligature and subdivides the body column into two developmental com partments. (4) The basal head in turn organizes a mirror-image duplica tion of the body pattern. In pattern regulation, Hydra follows rules o f intercalation known from other organisms.