Because the embryos of polyclad turbellarians exhibit the ancestral features of quartet spiral cleavage and a trochophore-type larva, knowledge of polyclad development is crucial to understanding the evolution of development in the Spiralia. Surprisingly polyclads have been the subject of few developmental investigations, and experiments have been done only on the embryos of Hoploplana inquilina (see Boyer, 1995). However interpretation of the results of this work has been handicapped by the lack of a complete cell lineage of the embryo. Recently Henry et al. (1995) have shown that the quadrant fates of four-cell stage Hoploplana embryos are essentially identical to those of the higher Spiralia. The present study follows the cell lineage from the eight-cell stage through the fourth quartet. The first three quartets of micromeres form primarily ectoderm, including the nervous system. Mesoderm comes from second and fourth quartet blastomeres, with 2b giving rise to circular muscles and 4d to longitudinal muscles and mesenchyme. The remainder of the fourth quartet apparently serves as nutrient reserves for the developing embryo. This work demonstrates a striking similarity between the cell lineages of polyclad and higher spiralian embryos. It also confirms that there are both ectodermal (2b) and endodermal (4d) contributions to the mesoderm in primitive spiralians and supports the concept that the dual origin of mesoderm represents the ancestral condition.