The foundation of the germ layers and the extraembryonic mesoderm from the epiblast between 6.5 and 7.5 days post coitum (p.c.) is accompanied by substantial cell proliferation. It is followed during the next 24 hours by the organization of major systems of the embryo such as the central nervous system, somites, heart and vascular system. Injection in situ of a short-term lineage label (horse radish peroxidase) into single epiblast cells at 6.7 days p.c. and analysis of the descendant clones in cultured embryos have been used to trace these processes and led to the following conclusions: (1) There is extensive but not indiscriminate cell mixing at the onset of gastrulation; epiblast cells spread towards the primitive streak and descendants are there progressively incorporated into mesoderm. (2) The fate map of the mouse epiblast at the early primitive streak stage is topologically similar to those of other vertebrates. (3) Germ layers and the extraembryonic mesoderm are not clonally distinct before gastrulation, the region of overlapping boundaries in the fate map being occupied by cells that will have descendants in more than one layer. (4) Cranial neurectoderm is mainly derived from axial epiblast immediately anterior to the primitive streak of the early streak stage embryo, clonal descendants being spread rostrocaudally in the developing neural tube. Contribution to the putative floor plate is made by progenitors some of which also contribute to notochord and mesoderm.