Embryos of the nematode Caenorhabditis elegans were serially sectioned and photographed in the electron microscope (EM). The micrographs were used to produce threedimensional (3D) reconstructions. Size and position of each nucleus were entered into a computer, displayed as spheres, and were color-coded to indicate lineage membership. Location in space and position in the cell cycle are generally adequate criteria to identify cells. The reconstructions allow visualization of lineage-related topographic patterns and ultrastructural analysis of differently determined cells.