The chromatin subunit or nucleosome structure in the fertilized embryo of the sea urchin Strongylocentrotus purpuratus has been investigated during early stages of growth when the translation of mRNA for histone variants of HI, H2A, and H2B is markedly changing. It is shown that nucleosomes exist prior to differentiation, in cells which are "totipotent". We have found that micrococcal nuclease derived monomer and multimeric nucleosomes can also be isolated from each stage of development, beginning with the two-cell embryo. Basic proteins in the nucleosome fractions have been analyzed directly by employing a new technique developed in our laboratory: protamine release of chromosomal proteins into Triton-acid-urea mini gels [Shaw, B. R., & Richards, R. G. (1979) NATO Adv. Study Inst. Ser., Ser. A 21a, 125-136], These studies indicate that cleavage-stage proteins are nucleosomal proteins. Likewise, the a-type histone H2A variant and each of the putative late stage f), y, and 8 H2AIjittle is known as to how variations in histone subtypes other than HI affect the structure of chromatin. Multiple forms of histones H2A, H2B, and H3, which differ by one or several amino acids in their primary sequence, have been identified in a number of organisms (

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