PROPORTIONAL contributions of individual histone fractions to total histone and their degree of microheterogeneity change during development. This has now been established for many plants and animals1,2 including sea urchins3–6. The very lysine-rich histones, designated F1, can be fractionated as several proteins of similar amino acid composition7,9 and as different phosphorylated derivatives10,11. These histones in particular show considerable species specificity in primary structure, molecular weight, and microheterogeneity8,9,12,13. It has been demonstrated in earlier studies3–6 that aside from changes in proportion among histone classes synthesised during sea urchin development, synthesis of an F1 characteristic of the morula stage (F1-m) gives way to synthesis of a new and electrophoretically distinct F1 at the gastrula stage (F1-g). Significantly, the F1-m histone is neither degraded nor converted to F1-g during the interval from morula to gastrula6.