Polar morphogenesis in early hydroid development: Action of caesium, of neurotransmitters and of an intrinsic head activator on pattern formation.

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.