1. A modification of the Nadi or indophenol reaction, using very low concentrations of agents, and intracellular oxidation of dyes reduced by sodium hydrosulphite, demonstrates the existence of a cell-wall oxidation gradient, decreasing from the blastocoel outward and from the apical region basipetally, in normal blastulae and early gastrulae of Strongylocentrotus purpuratus, Dendraster excentricus and Patiria miniata.2. In the evaginated entoderm of exogastrulae, derived either from the original prospective entoderm or from entodermized ectoderm, the cell-wall gradient is reversed in direction, decreasing from the external surface toward the blastocoel.3. The cell-wall oxidation gradient of ectoderm which persists as ectoderm does not undergo reversal, but as the cell-wall decreases in thickness, it becomes difficult or impossible to determine whether it persists.4. Reversal of the cell-wall oxidation gradient by an exogastrulating agent is regarded as resulting from two factors: first, more or less comp...