The purpose of this module is to provide a survey of the rapidly expanding field of developmental biology and to introduce it to the student in a unifying way. In medical schools where courses in biochemistry, physiology, and pharmacology are already considerably intersecting, there is not surprisingly a rising demand in modern medical education for books emphasizing the interdisciplinary approach. In recent years, developmental biology has become a very vibrant and exciting field. The adoption of the interdisciplinary approach in this field has yielded enormous information about how DNA is able to produce a living organism from a fertilized egg. The discovery of 'master' genes in Drosophila that control spatial organization and share a segment of DNA, the so-called homeobox, and the discovery in C. elegans of genes controlling the timing of branching off of cell lineages are today recognized as milestones in molecular developmental biology. Because of space limitations and because of the information explosion, we have continued to pursue the policy of selecting broad topics but not in every case. This time, for example, though guided by the principle that a close connection exists between genes, adhesion, and morphogenesis, we opted to include certain topics such as cadherin - an adhesion molecule - rather than have the whole subject of adhesion dealth with in a single chapter. Substrate-adhering molecules (e.g., fibronectnin) are touched upon in Chapter 5. In a similar manner, only one type of junction is discussed at length. Chapters 8, 9, and 10 border on the extraordinary, for they are together absorbingly interesting. The last chapter makes things more pragmatic. The attention of the reader is drawn to the fact that several previous volumes of the compendium impinge on the present one. Chapters 25 and 26 in Volume 7B, in particular, have much to say on the subjects of extracellular matrix adhesion and intercellular communication.