Causal analyses of development are currently based firmly on "information" models. With the advance of molecular genetics it was eagerly believed that these models would be corroborated by observed gene - gene interactions and would sufficiently explain morphogenesis. However, the gap between the gene expression/protein level and morphological levels of phenotype could not be bridged. The suggested project will investigate epigenetic mechanisms in development and their contribution to phenotype design. These are mechanisms of interaction between different levels of organismic integration. It is proposed that the unit of interaction is the "unit of gene expression" and that the forces of interaction are mainly epigenetic. How directed changes of the phenotype take place, as units of morphological integration interact epigenetically, will be described by a mathematical model of the "interface space". To test the model it will be applied to epidermal development. The epigenetic effects of generic cell and tissue properties in skin patterning processes will be studied using my own experimental data. On this empirical basis, a formulation of general parameters of epigenetic mechanisms will be attempted.