AbstractThe developmental stages of 12Erythrocebus patasembryos, ranging in gestational age from 30 to 50 days, is described. The pattern of embryogenesis inE. patasclosely parallels the anatomic characteristics of human and other nonhuman primate embryos between stages 12 and 23. However, there is a delay in development inE. patassimilar to that observed in human embryos which differs from the macaques and baboons. This temporal difference in the embryonic period is an important factor in the design and analysis of early pregnancy studies in this species.