The influences of ecto- and endoderm in determining the axial polarity ofHydra attenuata Pall. (Cnidaria, Hydrozoa).

Ectoderm and endoderm of the gastric column ofHydra attenuata Pall. were separated from each other and reassembled with either the same (controls) or opposite polar orientation. The controls always regenerated a head and basal disc according to the original polarity. In those specimens in which the polarity of ecta- and endoderm was opposite 33 specimens out of 41 reconstituted a single polyp whose body axis was clearly identifiable. Of these cases 8 followed the polarity of the ectoderm, 3 obeyed that of the endoderm, and 22 formed axes perpendicular to the original longitudinal axis. In 5 cases 2 specimens regenerated from the reassembled specimens. It is thus demonstrated that axial polarity ofHydra is determined by both the ecto- and the endoderm.