Developmental biology has been transformed recently by discoveries in the fields of molecular biology, cell biology, and immunology. New ways of manip ulating mammalian development are uncovering control mechanisms and ena bling us to apply them in solving practical problems in animal production and human health. This book outlines some of these new manipulations and how they have contributed to the present state of developmental biology. Chapter 1 describes gene transfer by micro injection of cloned recombinant DNA into zygotes. Although the factors that affect transformation frequencies and integration sites are still unknown, such techniques offer a number of exciting prospects. Research models for human disease coula be artificially created and desirable characteristics in agricultural animals could be - hanced. . The theme of cell-to-cell transfer is continued in Chapters 2 and 3. Chapter 2 describes pronuclear transplantation by Sendai virus-induced fusion of the karyoplast with the enucleated embryo. Using this procedure, it has been dem onstrated that both male and female genomes are essential for normal develop ment, although the reason for this is not yet understood. Chapter 3 describes studies on the fusion of whole oocytes. .