Figure. 41.

Systematic grafting experiments revealed the importance of the provenience and the destination position of the tissue fragment. Head cells are supposed to segregate head inhibitors. Inserting a head fragment close to a head inhibits new head formation (a). Only at distal inserting positions a head is formed since the inhibitor level is low there (b). Grafts of distal origins also show only weak head formation even if inserted at a distal position as the activator level is small (a). (c) The activator activates head formation but at the same time raises the inhibitor level while the inhibitor in turn suppresses the activator and consequently the head formation [505,506]. Figures (a) + (b) were adapted from Müller [505] with permission from Elsevier.