From a single cell - a fertilized egg - comes an elephant, a fly, or a human. How does this astonishing feat happen? How does the egg ''know'' what to become? How does it divide into the different cells, the separate tissues, the brain, the fingernail - every tiniest detail of the growing foetus?These are the questions that the field of developmental biology seeks to answer.It is an area that is closely linked to genetics, evolution, and molecular biology. The processes are deeply rooted in evolutionary history; the information is held in genes whose vital timings in switching on and off is orchestrated by a host of proteins expressed by other genes.Timing is of the essence. Here, the distinguished developmental biologist Lewis Wolpert gives a concise account of what we now know about development, discussing the first vital steps of growth, the patterning created by Hox genes and the development of form, embryonic stem cells, the timing of gene expression and its management, chemical s