Abstract Reduction of blue tetrazolium, coupled to the oxidation of DPNH and TPNH, has been studied in developing sea urchin eggs. These reductions are found to appear in the same regions of the larvae as the two reduction gradients of Child. TPNH-dependent reductions are found to occur in all stages and trends of development if glucose-6-phosphate or citrate is added. DPNH-dependent reductions are observed in early stages if DPN is added, but in later stages, i.e. gastrulae and larvae with hypertrophied entoderm, regardless of the addition of DPN. The change in specificity of the tissue at the differentiation of the entoderm is thus followed by a change in the DPN requirement.