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This chapter discusses some recent evidence on cell differentiation, cell movement, cell interaction, and morphogenesis in hydra with particular emphasis on spatial patterns and prepatterns. It discusses some general concepts of the biological pattern formation, with emphasis on physicochemical mechanisms capable of forming reproducible spatial patterns. Spatial patterns of differentiated cells may result from the regulation of the time sequences and quantities, in which various cell types are produced. Hydra is a few millimeters long and has an asymmetrical polar organization with tentacles, hypostome, and gastric column, including the budding zone, peduncle, and basal disk. The most interesting feature of hydra is its high power of regeneration. The evidence on regeneration and transplantation shows that prepatterns are involved in directing the positions of new heads and feet. Prepatterns in hydra exhibit a set of features typical of many other biological systems. This lateral inhibition type can be accounted for by short-range activation, long-range inhibition, and certain kinetic conditions that lead, starting from near-uniform initial conditions, to the firing of stable self-regulating patterns that show a set of properties of many biological patterns.
