The yolk utilization during embryonic development was monitored in the marine crab Charybdis bicifera. Water content enhanced from 68.60% to 90.34 %; ash content increased from 1.9% to 19.02%; whereas, all the organic constituents declined with a range from 66.88% to 56.63% in protein, from 26.58% to 20.11% in lipid and 4.42 to 27.19% in carbohydrate. Of the organic substances, lipid was the major source of energy, since nearly 50% of the total energy was stored in the form of lipid in the eggs. The protein and carbohydrate contributed only 41.94% and 8.38% of the total energy respectively. During embryonic development, the saturated fatty acids increased by 13.73% but the monounsaturated and polyunsaturated fatty acids decreased by 8.03% and 8.05% respectively. Of the two essential polyunsaturated fatty acids, the 20:5n-3 decreased linearly and the 22:6n-1 maintained about a constantilevel throughout the embryonic development. Proteins in developing eggs are progressively depleted for the possible utilization during embryogenesis. The decrement of fat during embryogenesis not only provides metabolic energy but also for freezing the protein from being oxidized. The carbohydrate has a dual role of supplying energy for metabolism and contributing to structural formation.