StripesStripes are another periodic pattern that is frequently encountered in development. The zebra stripes are proverbial. Transitions between patch- and stripe-like patterns can be frequently seen on the skin pattern of tropical fishes. How can pattern elements emerge that have a long extension in one dimension but a narrow extension perpendicular to the first? According to the model, stripes can be formed if the rate of activator autocatalysis saturates at high activator concentrations. This leads also to a limitation of the inhibitor production. Since the activator production per cell is lower, more cells become activated until sufficient inhibitor is produced that abolish a further enlargement of the activated region. In other word, with saturation, the activated regions have the tendency to enlarge. However, in order to become activated, a close neighborhoodto non-activated cells is essential into which the inhibitor can be dumped. Both requirements, large activated patches and a direct nneighborhood of non-activated cells, seems to contradict each other. This is, however, not the case. In a stripe-like activation pattern, each activated cell has an activated nneighborand non-activated cells are close by. Kondo and Asai  observed regulating of striping patterns on tropical fishes and have shown that corresponding insertions and splitting can be explained using this type of model (for equation and parameters click here ).
Molecules involved the generation of periodic structuresIn the Drosophila fly the precursor cells of the peripheral nervous system (neuroblasts) are derived from two lateral bands of ectodermal cells. Crucial for neuroblast formation are the genes of the Achaete-Scute complex. Both genes have a direct autocatalytic feedback on their own synthesis [1,6]. Cells that express these genes activate the gene Delta that codes for a molecule exposed at the cell surface. It stimulates the receptor Notch, a molecule ubiquitously present on all cells in that region and that acts ultimately on the Achaete-Scute complex in an inhibitory way. In this way, particular cells are singled out to participate in the formation of the peripheral nervous system while the remaining cells form the ectoderm.. Similar reactions have been found in the spacing of hairs on the leaves of Arabidopsis . In the generation of highly regular feather patterns, BMP-2 and BMP-4 are involved in the lateral inhibition , while FGF-4 and sonic hedgehog are elements of the activating mechanism . In these investigations, the regions in which genes for activating and inhibiting molecules are transcribed were determined. As expected from the theory, in most cases these regions coincide. A more direct comparison with the theory, however, would not only require a knowledge of regions where the molecules are produced but of the actual ranges of the signaling molecules.