Metamorphosis and pattern formation in Hydractinia echinata, a colonial hydroid.

There are several reasons why Hydractinia echinata Hydrozoa, Cnidaria) is excellently suited to study developmental processes. In the laboratory fertilization takes place every morning in the seawater in thousands of eggs. Cleavage starting synchronously leads to a ciliated planula larva within 2 to 3 days. Onset of metamorphosis from the larval to the polyp stage must be triggered externally. There are several agents known to induce or to interfere with induction of metamorphosis thus allowing access to the biochemical basis of this process. The pattern of the resultant polyp can be influenced by certain treatments during the process of metamorphosis allowing access to a process of proportioning. The colony develops by elongation of hollow tubes at the base of the polyps, termed stolons on which in more or less regular intervals new polyps emerge. Two (main) types of polyps are formed allowing to study spacing by lateral inhibition and lateral dependence of each other. In the present paper current data and hypotheses concerning all these topics are discussed.