The last ten years have shown a dramatic revolution in our understanding of early animal development. This new edition of the successful first edition describes the result of this revolution and explains how the body plan of an embryo emerges from the newly fertilised egg. The book starts with a critical discussion of embryological concepts and explains in simple terms the mathematics of cell states, morphogen gradients and threshold responses. The experimental evidence on the mechanism of regional specification in Xenopus, molluscs, annelids, ascidians as well as Caenorhabditis, the mouse, the chick and Drosophila is then discussed. The whole chapter devoted to the exciting developments in Drosophila provides a clear guide to the subject, including a new table outlining the developmentally important genes. The emphasis throughout is on conceptual clarity and unity: bringing together the mathematical models, embryological experiments and molecular biology into a single, comprehensive coherent account.