Examines the establishment of the germ layers and other cell lineages in the early embryo including details of cell movements during the beginning stages of primitive streak formation. Discusses patterns of gene expression during the development of such tissues as the limb bud, skeletal, muscle and the central nervous systems placing special emphasis on commitment to particular cell types. Although it concentrates on the mouse as an example of mammalian development--chick, amphibian and Drosophila embryogenesis are employed whenever these organisms are more applicable to the study of a particular problem.