Wing buds with three ZPAs.
It has been hypothesized that a diffusible morphogen emanates from the ZPA of the embryonic chick limb and that cells can have their presumptive fate respecified to form more posterior structures when exposed to a higher concentration of morphogen. Attempts to raise the concentration of morphogen by grafting two additional ZPAs into a host wing bud failed. Instead, the greater the positional disparity between the donor and host tissue at the graft sites, the greater the number of extra structures that formed.
D000818:Animals; D002642:Chick Embryo; D008954:Models, Biological; D014184:Transplantation, Homologous; D014921:Wings, Animal / Q000196:embryology* / Q000637:transplantation