Figure 1. Diagrammatic representation of a freshwater planarian cut at different body levels (---) to visualize the different modalities of regeneration: 1, terminal posterior; 2, terminal bipolar; 3, terminal anterior; 4, lateral; and 5, intercalary. Arrows indicate the direction of the different regenerative processes. Green areas indicate the postblastema areas. Postblastema refers to the stump areas below the wound that by continuous neoblast proliferation and migration to the wound produces the blastema (blue). In intercalary regeneration (5), heads and tails are cut, the middle piece removed, and head grafted to tail. An intercalary blastema develops formed by undifferentiated cells from both head and tail pieces. a.PB, anterior postblastema; p.BP, posterior postblastema; L.PB, lateral postblastema; aB, anterior blastema; p.B, posterior blastema; L.B, lateral blastema; I.B, intercalary blastema; ey, eye; ph, pharynx; w, wound. Reproduced with permission from Ferretti P and Géraudie J (eds) (1998) Cellular and Molecular Basis of Regeneration. From Invertebrates to Humans. Chichester: John Wiley & Sons. Copyright © 1998 John Wiley & Sons Ltd.

Figure 2. Intercalation of positional values by growth in the regenerating cockroach leg. Left panels: a distally amputated tibia (positional value 5) grafted to a proximally amputated host (positional value 1) induces, regardless of the proximodistal orientation of the graft, the intercalation of the positional values 2–4. A normal tibia is regenerated. Right panels: a proximally amputated tibia (positional value 1) grafted to a distally amputated host (positional value 4) regenerates a longer than normal tibia with reversed polarity as judged by the orientation of surface bristles. The reversed orientation of regeneration is due to the reversal in positional value gradient. The proposed gradient in positional value is shown after each figure. Adapted from Wolpert LW (1998) Regeneration. In: Principles of Development, pp. 399–416. London: Current Biology. Copyright © 1998, with permission from Elsevier Science.

Figure 3. Supernumerary legs appear in many cockroaches when grafting experiments lead to abnormal cell confrontations. A distal half of a donor right tibia is grafted onto the stump of the proximal half of a host left tibia (a). Two supernumerary legs appear (b), one at each of the two points of maximum axial incongruity (V/D and D/V). Following the results obtained in regeneration of imaginal discs in Drosophila, and assuming similar patterns of gene expression and regulatory circuits in cockroaches, this manipulation will appose decapentaplegic-expressing cells and wingless-expressing cells at the new V/D and D/V boundaries. In this situation, distalless is induced at the boundaries and distalization (growth of a new leg distal to the level of the cut) ensues. After Bryant PJ, Bryant SV and French V (1977) Biological regeneration and pattern formation. Scientific American 237: 66–81. Copyright © 1977 Ikuyo Tagawa Garber.