During embryogenesis of the spider crab, Hyas araneus, four developmental phases were distinguished microscopically: cleavage, gastrula,;embryo differentiation, prehatching phase. Changes in fresh weight (FW), dry weight (DW), ash-free dry weight (AFDW), water, carbon (C), nitrogen (N), total lipids, proteins, carbohydrates, and respiration rates were measured in developing eggs. The percentage of water increased from 52% to 87% of FW, most rapidly after the beginning of organ differentiation and during hatching. The accumulation of minerals followed a similar pattern, with ash increasing from 3% to 7% of DW in the eggs, and to 26% at hatching. Inverse (decreasing) patterns were observed in organic matter (AFDW, C, N, lipids, proteins). During development from the undivided egg to the freshly hatched zoea larva, 69% of the initial lipid and 35% of protein were depleted. Carbohydrates represented a minor constituent of yolk (1% to 2% of DW). In spite of a decreasing protein content, N remained almost constant. This suggests a final increase in low-molecular nitrogenous substances, which are not quantitatively detected by the Lowry method (probably free amino acids); we suspect that these may play a role in the hatching process. In the gastrula, and from about 3 months after the onset of organ differentiation until 1 or 2 months before hatching, low embryonic respiration rates as well as microscopical observations indicated the existence of developmental resting periods. Such diapause-like intervals may coordinate the time of hatching with a short season of planktonic food production in high latitudes. The relative importance of lipids and proteins as fuel for embryogenesis is compared with that in other crustacean taxa.