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.