1. Stages in embryo formation among Amphibia between yolk plug and gill circulation have similar time-temperature relations.2. The time-temperature relation of cleavage, although constant from first to fourth cleavages, differs from that of embryo formation.3. It is suggested that the different time-temperature relations of cleavage and of morphogenesis represent different controlling processes; while the similarity of the time-temperature relations among cleavages and among stages of later development is the expression of a common controlling process in each case. These controlling processes are probably parts of the respiratory metabolism and they prevent temperature from disorganizing development.