Figure 1.

Abbreviated Hydra regeneration cycle. (Left to right) A mature Hydra is dissociated into individual cells of either ectoderm (green, outer tissue) or endoderm (purple, inner tissue) lineages that are subsequently concentrated into a heterogeneous aggregate. Upon aggregation, the ectodermal cells sort to the outside of the aggregate and the endodermal cells concentrate in the interior, creating a sphere-within-a-sphere. Following this, a variety of developmental steps occur that eventually result in a fully regenerated Hydra. To see this figure in color, go online.