Abstract Lipid metabolism is an important aspect of chick embryonic development because avian embryos derive over 90% of their caloric requirement from fatty acid oxidation (Boell, 1955). In addition, the embryo requires fatty acids for synthesis of phospholipids for membrane formation and for synthesis of triglycerides for energy storage. Several years ago we examined the fatty acid composition of embryonic lipids and were intrigued by the fact that although the differences were small, the fatty acid composition of embryonic lipids was consistently more saturated than the composition of the lipids of yolk from which the embryos developed (Table 1). There are three possible explanations for this: first, the embryo may selectively absorb saturated fatty acids from yolk for tissue deposition; second, the embryo may selectively oxidize unsaturated fatty acids; third, the relative synthesis of saturated fatty acids may be greater than that of unsaturated fatty acids. If one compares the fatty . . .