This book is about how an egg becomes an animal. Attention will be concentrated on early development because this is the time at which the important events are happening. As everyone knows, the human gestation period is about nine months long but it is not so commonly appreciated that the basic body plan of the embryo becomes established during the very short period from one to four weeks after fertilization. During this time an apparently homogeneous group of cells, the inner cell mass of the blastocyst, become transformed into a miniature animal consisting of central nervous system, notochord, lateral mesoderm, somites, pharyngeal arches, integument and gut. All of these parts contain specific types of cell and all lie in the correct positions relative to one another. In later development there is a good deal of growth and of histological differentiation of the organs and the specifically human, rather than the general vertebrate, characteristics of the organism become established. However, all this takes place on the framework of the basic body plan which was laid down in early development. As Wolpert has emphasized, it is not birth, marriage or death, but gastrulation which is truly the most important time in your life.