Mesodenn is a key tissue in early development. It is involved in the differentiation of almost every organ in the body, not merely as a structural component, but as an active participant in the establishment of diverse cell types. All mesodenn is derived from ectoderm. Its appearance signals the start of a significant new phase in the development of the embryo. At this time all three genn layers are now present and myriad sequences of cell and tissue interactions begin to occur which will eventually give rise to the entire embryo. The control of the growth and differentiation of the mesoderm is critical for the production of a normal individual. Indeed, disturbance of the patterning of the mesoderm or of its interaction with other tissues plays a critical part in the fonnation of most congenital anomalies. The main focus of this book is therefore on the establishment, divergence and specialisation of mesodermal derivatives. The central role of the mesoderm in development has long been appreciated and a wide literature exists on its activity in certain specialised situations. Recently, however, an impetus to its study has been provided by new approaches opened up through biotechnological advances. Many of these advances are reflected in the reports in this volume. Scientists from various disciplines have become drawn to mesodermal tissues, and this volume may help them find a framework within which their work will fit.