Fig. 9. A model for position-dependent regulation of regenerative growth rate. (A) After amputation, yet undetermined signals recognize position and establish cellular identity. These signals are thought to be present in a gradient along the PD axis, and introduce position-dependent properties of Fgf signaling. (B) The amount and PD length of Fgf signaling, each of which are greater in proximal regenerates, determine the PD length and the mitotic index of adjacent blastemal mesenchyme (blue). Because there is little overlap between highly proliferative blastemal tissue, regions of active Fgf signaling in the epidermis (green) and the distal region of the blastema (red), it is likely that an Fgf-dependent epidermal mitogen mediates blastemal proliferation (arrows). Greater influences by Fgf signaling on blastemal cells of proximal regenerates lead to higher growth rates than distal regenerates.