When W. Roux reported the results of his manipulations of early frog embryos in 1888, he ushered in a new era of inquiry into the nature of living things. In short, Develop mental Biology had been born as an experimental science. Until that moment the path to discovering the processes that control the emergence of form had lain hidden beneath a pile of descriptive and comparative embryology that accumu lated throughout XIX century biology. This approach though necessary as a prologue to what would follow was. in isolation, fatally barren in providing real insights into the causes of the processes that it describes. Shortly after pub lishing the results of his experiments, Roux founded the Archives für Emwicklungsmechanick der Organismen and launched a crusade for experimental interference with devel oping systems of all kinds and at all stages. A few years later H. Driech published the results of a series of experiments in the spirit of the new era. By tinkering with blastomeres of early sea urchin embryos. Driesch revealed the extraordi nary regulatory capability of cell assemblies; for the first time. the central role hitherto ascribed to lineage in deter mining the shapes and forms of living systems was called into question and the seeds of what is now known as the concept of inductive interactions was sown. There followed a steadily growing catalogue of more or less sophisticated observations of such regulatory abilities and their constraints. These observations represented the first steps towards a causal analysis of normal development, but alone they could never deliver a true mechanistic understanding of the processes they revealed. Yet for a long time, devel opmental biology seemed hidebound by this approach, its attention fixed firmly on the study of embryos placed, as Spemann once put it, into ever more emban-assing situations.
Nonetheless, slowly these alternative approaches permeated this cataloguing and tinkering and they slowly started to shed light on the causes of the wonderful behav iours observed during the experimental phase. The last decade has been particularly fruitful in the understanding of the mechanisms that underlie the phenomena first observed by Driesch. Development has come to be viewed in terms of the coordinated expression of genes in time and space. Central to this view, and still ill understood, lie the signalling systems that orchestrate such coordination. Against some expectations it has become clear that the clockworks that underlie developmental mechanisms are not very different from those that underlie the workings of single cells. In this sense, one of the achievements of the last few years has been the rephrasing of many questions of old, in terms of molecular biology and genetics. The answers that have arisen have made clear that what developmental biology deals with is cell biology with an interesting twist: instead of problems in "one-cell biology" they are problems in "many-cells biology".
The rate of progress has been asLOnishing and even since the publication of The Molecular Basis of Positional lnfor mation (Development 1989 Supplement), huge advances have been made. Then (only four years ago!) the questions focussed on the types and possible identities of signals that may operate in different systems. Now we are gaining a detailed understanding of entire pathways by which many of these signals are relayed and interpreted by the nuclei of the cells, as well as of the outputs that they elicit. With this knowledge, new horizons come into view, so that we can now consider the ways in which cell shape, behaviour and organization are influenced by the many signals they receive. This analysis has required a convergence of knowledge and ideas from a variety of disciplines, which has facilitated progress. The contributions to this volume reflect this blending process and at the same time highlights the power and influence of genetics in unifying the paths of cell and developmental biology towards a common goal. In the spirit of this unification and in contrast with previous volumes of this series we have not attempted subdivisions into distinct sections. There are themes: cell signalling, adhesion and polarity, which delineate the continuity of their particular contributions to the seamless continuous process that is the generation of living form.
