ABSTRACT
1) Polar pattern formation and morphogenesis during metamorphosis ofHydractinia echinata are pre-programmed by the spatial distribution of morphogenetic potencies along the body axis of the planula larva. This prepattern becomes manifest when parts of larvae are brought to metamorphosis immediately after isolation. The anterior region forms only stolons, the posterior only a head. The middle region can develop stolons as well as heads. The potencies for head and stolon formation establish gradients with opposite directions. 2) The origin of this pattern during embryogenesis and its expression in the course of metamorphosis can be influenced by chemical stimuli. Two alternative effects can be obtained: activation of head formation at the expense of stolonal growth (=oralization); hyperdevelopment of stolon tissue at the expense of head structures (=aboralization). 3) Caesium ions temporarily applied during cleavage result in larvae which complete metamorphosis with symptoms of aboralization. Cs+ ions applied during early metamorphosis (disc stage) exert an oralizing influence. The sensitive periods coincide with phases of high Na+-K+-ATPase activity. Ouabain abolishes the morphogenetic action of Cs+. 4) Strong symptoms of oralization or aboralization can also be evoked by exposing disc stages to neuropharmacological agents. Acetylcholine (carbachol) brings about oralization; serotonin causes aboralization. 5) In planulae a substance can already be traced which corresponds to the neurosecretory head activator known from adult organisms. Along with initiation of metamorphosis, head activator activity rises steeply and already approaches its final level in early metamorphosis. Activities extracted from metamorphosing larvae or various adult Cnidaria when applied to disc stages promote head formation without affecting stolonal growth. Dose response curves display optimum peaks. 6) The significance of pre-programming and of neuroid functions in polarized patterning are discussed.
