Three dimensional avian limb bud growth. A chemical FGF has been superimposed on the grid with a piecewise linear distribution in the z direction. FGF concentration determines if cells are in the apical ectodermal ridge (AER) within which no reaction-diffusion can occur (red cells). The rest of the cells are undergoing haptotaxis to an external chemical Fibronectin, which is secreted by cells. They are either of type Condensing (green) or NonCondensing (blue), depending on the activator chemcial concentration at the location of each cell. Note: Condensing cells have been superimposed on the grid to create a clearer picture.