In the simulation shown above, the endoderm (red) forms by a pattern forming reaction. This gene activation is connected with a suppression of genes responsible for ectoderm formation (blue). In the absence of activator diffusion (community effect), the endoderm cells appear in an intermingled arrangement.
Separation of the germ layers: zones or individual cells?
In amphibians, well-separated zones with endo- and mesodermal specification, respectively, are formed before gastrulation begins. In the zebra fish, both cell types are more intermingled and the progeny of a single cell can give rise to both cell types, indicating that the pattern formation is still going on [5,6](Warga and Nüsslein-Volhard, 1999; Rodaway et al., 1999). Similarly, in the chick, individual cells separate from the epiblast and ingress to form meso- and endoderm. In some Cnidaria
including hydra, the intermingled determination of ecto- and endoderm seems to be even more extreme. First a single layered coeloblastua is formed from which individual cells delaminate and ingress to form later the endoderm of the gastric cavity  . Although molecular data are not yet available, this behavior suggests that the choice between the two pathways is made first in a salt and pepper-like fashion in the single-layered cell sheet, followed by an individual ingression of the endodermal cells.
According to the model, whether differentiated cells emerge as coherent regions in zones or individually on a cell-by-cell basis depends on the following factors. If the self-enhancing reaction can spread, large coherent patches are expected (community effect). If, e.g., by maternal determinants or preceding pattern forming reactions a preferential initiation site exists, such a zone appear at predictable positions. Alternatively, if the feedback loop does not involve secreted molecules or if their spread is limited, the decision is made on a cell-by-cell basis and random fluctuations can be decisive. Again, a condition for the salt and pepper pattern is a saturation of the autocatalysis, otherwise each cell will be surrounded by a halo of inhibition. All intermediate patterns between zones and salt-and-pepper distributions are possible under appropriate conditions. The ratio at which different cell types emerge is regulated.