Embryonic lipids exhibit a more saturated fatty acid composition than the yolk lipids from which the embryo develops. Neither selective embryonic uptake of saturated fatty acids from yolk nor selective embryonic oxidation of unsaturated fatty acids appears to be a valid explanation for this observation. Data are presented that suggest that fatty acid synthesis occurs in embryonic tissue, resulting in the production of saturated fatty acids in small but measurable quantities during development. Because embryonic tissue lacks the capacity to desaturate fatty acids synthesized de novo, fatty acid composition of the embryonic lipids is always more saturated than composition of yolk lipids.