Biologists at Tufts University have developed a computational model of planarian (flatworm) regeneration that explains how fragments of planaria determine which end should form a tail and which should form a head. The development begins to answer an important question in regeneration research—what are the signals that determine the rebuilding of specific anatomical structures? Combining modeling and experiment, the researchers determined that the direction of nerve fibers sets the redistribution of chemical signals establishing the direction of the head-to-tail axis. The model was also able to predict the outcomes of numerous genetic, pharmacological, and surgical manipulations, such as worms with two heads or two tails.