Terminal differentiation of head- and foot-specific epithelial cells occurs at the same location in Hydra tissue without polarity.

The reappearance of terminally differentiated ectodermal epithelial cells was studied in reaggregates of Hydra cells. These cells first occur separated from undifferentiated gastral tissue in mixed clusters consisting of cells which in normal animals are restricted to opposite body poles. Tentacles containing foot-specific basal disc cells as well as feet containing head-specific battery cells were formed from these clusters. This indicates that a positive cross-reaction of head- and foot-forming mechanisms exists at the cellular level and that induction of terminal differentiation precedes the establishment of polarity.