This book is an enquiry into the mechanisms by which the spatial organization of an animal emerges from a fertilized egg. It is intended for all students, teachers and research workers who are interested in embryos.

It is divided into three parts. The first two chapters introduce the problem of regional specification and attempt to define the meanings of embryological terms which are used in the remainder of the book. This is necessary because terms such as ‘induction’, ‘regulation’ or ‘polarity’ are often used but rarely defined and many controversies have arisen as a result of unnecessary misunderstandings.

The next four chapters give an overview of the experimental evidence which bears on the processes of cellular commitment from the time of fertilization to the formation of the general body plan. The animal types considered are those on which most experimental work has been done: amphibians, insects, other selected invertebrates, the mouse and the chick. This is a general survey rather than a detailed review but sufficient references are provided to enable interested readers to pursue the topics in greater depth.

The last four chapters attempt to generalize the problems and to investigate the extent to which they have been solved by the theorists and the model-builders.