Spacing patterns are of fundamental importance for various repeated structures which develop at regular intervals such as teeth, digits, and insect ommatidia. The chick skin develops a highly regular hexagonal feather pattern, a model system to study how spacing patterns develop; particularly since it can be studied in organ cultures. An electron-microscopy study showed that the first sign of feather bud formation is a thickening of the ectoderm, the epidermal placode, followed by dermal condensation. With a novel feather reconstitution procedure using an intact epithelium and reaggregated mesenchymal cells, periodically arranged feather buds form without the presence of primary row and sequential propagation. X-ray irradiation leads to increase in spacing and to a decrease in cell density in mesenchyme. Chemical agents such as Nocodazole, Cytochalasin B and Cytochalasin D has a similar effect. However, the spacing in midline is not affected by X-ray irradiation and chemical agents. Gene expression and signalling during early patterning of chick feather buds was investigated. Expression of a epidermal stripe of Sonic hedgehog and Fibroblast growth factor-4 is the earliest event detected in the dorsal midline of feather bud formation and local application of pre-soaked beads showed that Sonic hedgehog and Fibroblast growth factor-4 can induce the formation of feathers at specific times and positions and that Bone morphogenetic protein-2 strikingly inhibits feather bud formation uniformly. It is possible to specify periodic patterns by both a gradient of competence and lateral inhibition. The features of this model are a gradient of competence which changes with time, activators and inhibitors. As soon as the gradient of competence allows cells to respond to the activators, the first feather bud is initiated resulting in the synthesis of inhibitors. An activation zone is formed immediately adjacent to the initiation site with an inhibitory zone outside it, setting up the spacing pattern. A computer program based on these principles is submitted as part of this thesis.