This volume, the first in the MIT Cell Monograph Series, offers a thorough overview of a few selected topics in mammalian embryogenesis that are of current interest and interrelate well with each other. Most of the studies described deal with the mouse embryo, since laboratory maintenance of mice is convenient and the genetics of the mouse are well established. One of the underlying themes throughout the chapters is the programming of events in early embryogenesis. The activation of "new" genes, which begins early in embryogenesis, complements preexisting gene action and helps to provide the developing embryo with the machinery necessary for DNA, RNA, and protein synthesis, and with the enzymes required for intermediary metabolism ("maintenance" gene products). The synthesis of "differentiative" gene products, those which irreversibly route cells along different developmental pathways, must also be taking place prior to, or at, implantation. These new gene products, and what we know of their functions, are discussed in detail. Another, less understood area explored is the role of the genes that initiate differentiation in early embryonic cells. Recent biological studies have revealed that a relationship exists between the position of a cell relative to other cells in the embryo and its ultimate fate; however, embryologists have yet to explain how the microenvironment can trigger a cell to embark on a specific course of differentiation. Various methods used in investigating this problem—such as microanalytic techniques or the study of similar phenomena in teratocarcinomas—are described, and the relationships between embryos and teratocarcinomas are thoroughly considered. A chapter on T mutations in the mouse has been included because of the potential of this system for contributing to our knowledge of the mechanisms involved in development of the mammalian embryo. The use of recent immunological, histochemical, and biochemical studies of cell surface properties as markers of genetic programming during embryogenesis is described; and a chapter on the possible role of viruses in the control of embryonic development completes the volume. "In summary," the editor states, "the chapters in this monograph demonstrate that mammalian embryologists are now equipped with morphological, biochemical, and molecular biological markers of early embryonic development and differentiation. Model systems are available for testing ideas of genetic and epigenetic control mechanisms. With proper use of these resources, our understanding of mammalian embryogenesis could be revolutionized within the next decade."