Specification of the head-body proportion inHydra attenuata regenerating the head.

Pattern formation has two aspects: a structure must be formed at the correct position and be of the correct size. InHydra the inherent polarity of the tissue controls at which ends of a body segment the head and foot are regenerated. It remains unclear how it is controlled that structures of the correct size are formed. The head ofHydra attenuata, i.e. the tissue of the hypostome and the tentacles, contains about 22% of the animals epithelial cells. Both small and large fragments ofHydra tissue regenerate a head which preserves this proportionality. The process which determines the final size of the head starts in the first few hours after head removal. As the process of regeneration proceeds, a progressively larger part of the regenerating tissue is specified to transform into a head. This process continues until the volume of tissue with head specification attains the correct proportion with respect to the rest of the body. The foot-forming system interferes with the head-forming system: if foot regeneration starts before head regeneration, the new head becomes abnormally large.