Nearly 10 years have elapsed since I finished writing the first edition of Intro duction to Molecular Embryology. During this period, molecular embryology has made great strides forward, but without undergoing a major revolution; there fore, the general philosophy and outline of the book have remained almost un changed. However, all the chapters had to be almost completely rewritten in or der to introduce new facts and to eliminate findings which have lost interest or have been disproved. There was a major gap in the first edition of this book: very little was said about mammalian eggs despite their obvious interest for mankind. Research on mammalian eggs and embryos is so active today that this important topic deserves a full chapter in a book concerned with molecular embryology. Therefore, I am very thankful to my colleague Dr. Henri Alexandre, who has written a chapter on mammalian embryology (Chap. 9) and has prepared all the illustrations for this book.

The main questions that embryology has always tried to answer are the following: How can the fertilized egg, which has received a nucleus from the mother and another from the father, give rise to all the organs present in the adult? How is it possible, at a given time of development and in a special region of the embryo, that a limited number of cells can differentiate into muscles or red blood cells? It is the purpose of molecular embryology to answer such questions in terms of the properties of macromolecules. Although we are still very far from obtaining a complete answer, spectacular progress in molecular biology has allowed us to state the same problems in simpler terms. For instance, the word "nucleus" can be replaced by "a set of genes" or still better, by "deoxyribonucleic acid (DNA)"; the fertilized egg has, in fact, received equal amounts of paternal and maternal DNA and this constitu ent is really the most important part of the nucleus, since it contains, in its own molecules, all the program for development into an adult. As for the second question, biochemistry tells us that muscle cells would fail to contract and would not display their characteristic structure under the microscope if they did not contain considerable amounts of specific proteins: the so-called "contractile" proteins, actin and myosin.

Most people have some interest in embryos; this probably results, in part, from their interest in understanding the biological origins of themselves and their offspring and, increasingly, concerns about how environmental change such as pollution might affect human development. Obviously, et- cal considerations preclude experimental studies of human embryos and, c- sequently, the developmental biologist has turned to other species to examine this process. Fortunately, the most significant conclusion to be drawn from the experimental embryology of the last two decades is the manner in which orthologous or closely related molecules are deployed to mediate similar - velopmental processes in both vertebrates and invertebrates. The molecular mechanisms regulating processes fundamental to most animals, such as axial patterning or axon guidance, are frequently conserved during evolution. (It is now widely believed that the differences between phyla and classes are the result of new genes, arising mostly by duplication and divergence of extant sequences, regulating the appearance of derived characters. ) Other vertebrates are obviously most likely to use the same devel- mental mechanisms as humans and, within the vertebrate subphylum, the - parent degree of conservation of developmental mechanism is considerable. It has long been recognized that particular vertebrate species offer either d- tinct advantages in investigating particular stages of development or are - pecially amenable to particular manipulations. No single animal can provide all the answers because not all types of experiments can be carried out on a single species.

Edelman links genetics, development, behavior, and evolution.

Molecular Embryology explains in simple terms the molecular interactions that transform an egg to a complex embryo that in the end gives rise to a fully-formed animal. In doing so, the book covers one hundred and fifty years of experiments that have led to our present understanding of these molecular interactions. As the text progresses, the reader will gain a sense of the developmental similarities and differences between organisms. Students studying developmental biology and embryology will find this book an extremely useful introduction to the subject and will also appeal to anyone with an interest in the most recent advances in this largely undiscovered territory.

Molecular Embryology explains in simple terms the molecular interactions that transform an egg to a complex embryo that in the end gives rise to a fully-formed animal. In doing so, the book covers one hundred and fifty years of experiments that have led to our present understanding of these molecular interactions. As the text progresses, the reader will gain a sense of the developmental similarities and differences between organisms. Students studying developmental biology and embryology will find this book an extremely useful introduction to the subject and will also appeal to anyone with an interest in the most recent advances in this largely undiscovered territory.