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.