SYNOPSIS. A number of protozoa may have amoeboid, flagellated, or intergrade forms. At the present time several mechanisms have been proposed for inducing the formation of each of the above forms, but a definite triggering mechanism has not been elucidated. However, some change in the environment precedes the transformation of the cell from one form to another. Jahn (1962) and Czarska (1964), respectively, correlated ciliary reversal and water expulsion vesicle activity with alterations in the ionic environment. In both cases the processes involved are correlated with changes in the Gibbs‐Donnan (G‐D) relationship rather than direct ratios or molarities. It seems reasonable to assume that additional environmentally induced phenomena may also be based on changes in the relationship. The assumption is here made that an amoeboid cell, possessing the necessary genetic and physiologic potentials, can respond to certain changes in its environment by enflagellation. The following hypothesis is being considered: a change in the environment that increases the relative concentration of associated divalent cations is perhaps one of the main triggers for amoeba‐to‐flagellate transformations. Thus, in accordance with the G‐D theory, this transformation would be expected to occur when a given ionic environment is diluted. In addition, the transformation is discussed in relation to pH, population density, and other environmental parameters that alter the Gibbs‐Donnan ratio.

The hypothesis that the ameba-to-flagellate transformation (AFT) of Naegleria gruberi (NB-I) requires Ca 2+ and/or Mg 2 ' was tested with divalent cation blockers and competitors. Several compounds were found to decrease the rate and magnitude of the AFT as well as increasing the time of enflagellation onset. The compounds include: the chloride salts of lanthanum, cobalt, and manganese; C-6oo; and EDTA. These results suggest that the AFT requires the influx of Ca 2+ and/ or Mg 2 +.

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