Host‐Integrated Development in the Amoebidiales *

SYNOPSIS. Induction of the resistant stage in the Amoebidiales appears to be dependent upon a morphogenetic factor derived from the host animal. This triggering substance is released either at death or ecdysis of the host arthropod. Growth of the epizoon, Amoebidium parasiticum , in both complex and defined media, follows a sporangiospore sporangium pattern. The amoeba‐cyst phase, however, was only noted after crushed host material (Cladocera) was added to the axenically grown thalli. Subsequent tests, evaluated with a quantitative assay, indicate that amoebagenesis is influenced by pH, temperature, condition of the protist, and concentration of the active principle. Commercial “dried daphnia” has consistently yielded an active extract. A large number of complex and simple materials has been screened for activity, and only extracts of ground crab show promise as an alternate source. The amoebagenic factor is heat stable, relatively small, and not soluble in ether, acetone or chloroform. Paper chromatography with polar solvent systems yields a single active band, and this is the best evidence that amoebagenesis is induced by a single, rather than a multiple factor. Paramoebidium , the second genus in this obscure order, undergoes amoebagenesis at ecdysis of the host. The possibility that Amoebidium and Paramoebidium are simply different growth forms of the same protist is discussed.