Autoradiographic analysis of gastrulae of the sea urchin, Lytechinus pictus, after brief incubation periods with 3H-amino acids has indicated that the cells of the ectoderm incorporate significantly greater levels of isotope than the cells of the interior. Further analysis based primarily on incorporation patterns of exogastrulae indicate that the greatly lowered incorporation by the cells of the archenteron reflects the inability of small molecular weight precursors to reach the invaginated cells. The maintenance of high levels of 3H-amino acid incorporation by ectodermal cells of exogastrulae indicates their relative independence with respect to macromolecular metabolism from the cells of the archenteron.