A fate map for the cells of the early Ilyanassa obsoleta (Gastropoda; Mollusca) embryo will be constructed by tracing cell lineages. This will be done by the micro-injection of fluorescent dextrans using iontophoresis into individual micromeres produced in the third through sixth cleavages. The resulting pattern of fluorescence in organs of the one week old veliger larva will then be analyzed. The embryology of I. obsoleta is of considerable interest concerning mechanism of differentiation. These embryos form polar lobes, anucleate protrusions that shunt vegetal pole material to one cell after the first two cleavages. Polar lobes are one of the most classical examples of the cytoplasmic localization of morphogenetic determinants. In addition, evidence indicates that through inductive interactions the polar lobe regulates the development of structures such as eyes, which are formed from cells that do not contain polar lobe material. A complete lineage map specific to this embryo does not exist but is essential for the interpretation of previous experiments designed to probe the mechanisms of polar lobe function. The lineage technique will also be used directly to understand the development of experimentally-manipulated embryos. The origin of shell and parts of the digestive tract will be traced in lobeless embryos, from which the polar lobe has been removed. The origins of eyes and foot will be determined in larvae from partial embryos, which are composed of various blastomere combinations. The origins of duplicated structures such as eyes, shell, foot, statocysts, heart and intestine will be followed in equalized embryos, which have been manipulated so that at first cleavage polar lobe material is distributed to both cells, rather than one. A central question in analyzing all three experimental situations will be the extent to which determinant localization or inductive interactions are involved in the specification of individual cell fates.