The application of molecular biological techniques to the study of development has had a profound effect on our understanding of developmental processes. This 1985 book expounds some of the views on which hierarchies of genes control aspects of animal development. There is an emphasis on the best-studied systems, namely Drosophila and the nematode Caenorhabditis, though some features of mouse and sea-urchin development are also discussed. The book focuses on how information obtained from the study of mutants, combined with molecular details of the structure, regulation and functions of the genes involved, sheds light on the complex processes of animal development. The 1990 second edition, which is reprinted here, is an expanded and largely rewritten version of Dr de Pomerai's successful first edition. It features extensive additions to the chapter on insect development, a chapter discussing nematode development and another addressing differentiation in vertebrate systems.