1. The metamorphosis of the planulae ofHydractinia echinata (Hydrozoa) is induced by certain marine, gramnegative bacteria which at the end of the exponential growth release a stimulating principle. 2. The stimulus is liberated by stationary cells previously cultivated at low population densities (up to 10 cells/ml) in a proper medium (e.g. extract of meat). Transfer into seawater lacking nutritive sources enhances the inductive capacity. 3. The concentration of the inducing agent normally surpasses the threshold level only in the close microenvironment of living cells. But when shocked by a drop in the osmotic pressure the bacteria discharge increased amounts which become traceable in the filtered cell-free medium. 4. Thus the inducer can be accumulated and isolated by a process of osmotic shock which does not affect the viability of the microbes. The principle belongs to a category of microbial substances which are subsumed under the comprehensive term "leakage"-products. 5. The active principle can be precipitated from the leakage solution with acetone and extracted with chloroform. The inducer seems to be an unstable, nondialyzable, polar lipid. 6. In order to evoke complete metamorphosis the isolated agent must be applied in a pulse-like fashion. Using the onset of metamorphosis as criterion for the velocity of reaction the dose-response curves display Michaelis-like saturation kinetics. At short pulses the percentages of induced metamorphoses yield a saturation curve as well. This indicates that an enzyme or carrier-system is involved in the larval response. 7. The inducing effect of the bacterial principle is antagonized by ouabain. Conversely, high doses of the isolated leakage material abolish the ouabain inhibition. The primary effect of the inducer, therefore, can be interpreted as stimulation of the active cation transport, especially of the Na/K-ATPase.
