1. Differential inhibition of planula development can be brought about in the hydrozoan, Phialidium gregarium, by KCN, LiCl, ethyl urethane HCl (probably CO2) and by the presence of the medusae (CO2). Resulting modifications range from slight increase and extension of basal thickening of blastula wall followed by development of slightly modified planulae to spherical, solid, apparently apolar forms. In these immigration and mass ingrowth are increased and take place from all parts of the wall, and further development occurs only if a new polarity arises.2. In differential acclimation in low concentrations of inhibiting agents and differential recovery after return to water further development of persisting axes may occur or new polarities may arise through differential exposure of the surfaces of apolar forms or through "adventitious" localization of regions of greater activity. In rapid acclimation or recovery such axes may develop directly as hydranth-stem axes, but usually they are at first still more or...