1. Chemical composition and caloric content of egg and zoea of the hermit crabEupagurus bernhardus have been determined and the cumulative efficiencies of yolk utilization calculated. 2. The cumulative efficiencies of yolk utilization for different constituents were 70.0%, 55.3%, 79.4% or 35.0% for dry weight, caloric content, protein or fat. 3. Since the efficiencies with which the different constituents of yolk utilized vary, the chemical composition of the developing eggs undergoes considerable changes. While fat and caloric content showed remarkable decreases, water, ash and protein contents of the eggs increased. 4. Of 0.0722 cal expended on metabolic processes of the embryo, only 28.4% was drawn from the oxidation of protein, while fat contributed as much as 66.6%. 5. Considerable quantities of inorganic salts (0.77µg/egg) are absorbed from the sea-water by the developing egg.