Figure 1.

Fundamental questions of large‐scale patterning in planaria. (A) Global decision‐making in planaria. After bisection, the two resulting blastemas must make very different structures: a tail (A′) or a head (A″). However, the two sets of cells were at the same position in the worm and thus start out with the same information prior to the cut. This illustrates the global nature of regeneration, because the blastema needs information from the rest of the fragment to correctly make the AP anatomical fate decision. (B) Mixing neoblasts to probe target morphology. Schmidtea mediterranea (left, rounded head) and Polycelis felina (right, pointed head) are planarian species with different head morphologies. (C) Half the neoblasts of the rounded‐head worm are killed using irradiation and a lead shield. (D) Half the neoblasts of the pointed‐head worm are transplanted to the rounded‐head worm. (E) After the neoblasts have diffused, the head of the rounded‐head worm is amputated. Without a model of how target morphology is determined, it is impossible to predict what shape will regenerate, or if indeed regeneration will ever stop (given that neither set of neoblasts will be able to achieve their normal target morphology).