The fate map of 2- and 4-cell-stage Sicyonia ingentis embryos was determined by microinjection of lysyl-rhodamine -dextran into single blastomeres. Microinjected embryos were cultured to the limb bud stage, when the body plan of the nauplius larva was evident. The animal blastomere, AB, gave rise to anterior ectoderm, while the vegetal blastomere, CD, gave rise to posterior structures, including the invagination site during gastrulation. The A blastomere gave rise to mirror-image patterns of dorsal-lateral ectoderm, while the B blastomere gave rise to anterior, ventral ectoderm. The C blastomere gave rise to posterior, dorsal-lateral ectoderm, complementary to the A pattern, as well as some naupliar mesoderm. The D blastomere gave rise to mesendoderm, naupliar mesoderm, and some posterior ectoderm. To study the specification of the early blastomeres, they were microsurgically separated and cultured in isolation. Two mesendoderm cells formed in 1/2, 1/4, 1/8 and 1/16 blastomeres in embryos dissociated at the 2-, 4-, 8-, and 16-cell stages, respectively. CD and D blastomeres could be distinguished by their larger size and gave rise to the mesendoderm cells. Archenteron formation and elongation of the embryo occurred in CD but not in AB isolates. Isolated blastomeres were recombined in various ways to determine whether their state of commitment could be altered in different cellular environments. Duplicated mesendoderm cells and archenterons formed in CD + CD recombinations, while AB + AB recombinations formed blastulae but did not produce mesendoderm cells and did not invaginate. The normal number of mesendoderm cells and a single archenteron formed in D + AB recombinations, while C + AB recombinations remained as blastulae and did not form mesendoderm cells. The results suggest that the mesendoderm cells are autonomously specified, possibly by cytoplasmic localization at the vegetal pole. The mesendoderm may also function as a signaling region to organize other developmental events.