The embryonic cell lineage of the shoot meristem in maize (Zea mays L.) has been characterized using clonal analysis. Although there is considerable variability in the size and distribution of somatic sectors induced at a given time in embryogenesis, the fate of meristematic initials is not entirely random. During embryogenesis the number of cells in the presumptive shoot meristem increases and their fate becomes progressively more restricted. Cells toward the periphery of the presumptive meristem give rise to the lower nodes of the plant, while central cells form more distal nodes. Cell lineages become restricted to single nodes starting at the base of the plant, indicating that restriction of cell fate progresses from the periphery towards the center of the meristem. The number of twin shoots produced by X irradiation declines dramatically between 8 and 10 days after pollination, and is preceded by an acropetal progression in the level at which twinning occurs. This phenomenon suggests that the shoot meristem becomes determined gradually and that this process is completed between 8 and 10 days after pollination; i.e., just prior to or during the transition stage of development. At this stage the shoot primordium consists of 100–200 cells encompassing two or more cell layers of the embryo. The orientation of sectors that saddle the shoot (extend from one side of the shoot to the other) demonstrates that the shoot meristem arises from a region of the embryo containing longitudinally oriented cell files and that this cell pattern is at least partially preserved during shoot initation.