  We utilize whole-body Hydra regeneration from a small tissue segment to
develop a physics framework for animal morphogenesis. Introducing experimental
controls over this process, an external electric field and a drug that blocks
gap junctions, allows us to characterize the essential step in the
morphological transition - from a spherical shape to an elongated spheroid. We
find that spatial fluctuations of the Ca2+ distribution in the Hydra's tissue
drive this transition and construct a field-theoretic model that explains the
morphological transition as a first-order-like phase transition resulting from
the coupling of the Ca2+ field and the tissue's local curvature. Various
predictions of this model are verified experimentally.
