This textbook has been written by one of the most influential development biologists of the 20th century, Lewis Wolpert. Developmental biology is at the core of all biology and deals with the processes by which the genes in the fertilized egg control cell behaviour in the embryo and so determine its pattern, its form, and much of its behaviour. This book emphasizes the principles and key developments in order to provide an approach and style that will appeal to students at all levels.