Our objective is to determine the nature of the shifts in mRNA abundance during embryonic development of the sea urchin and the mechanisms through which developmental shifts in the abundance of specific mRNA are achieved. By hybridization kinetics with cDNA to polysomal poly(A) ion RNA, we have estimated that the most abundant mRNA species present in the mesenchyme blastula can be categorized on the basis of their degree of abundance in the gastrula polysomes. We have prepared a library of cDNA clones from the polysomal RNA of the mesenchyme blastula, which we have begun to screen for cloned mRNA sequences that display an assortment of developmental changes in abundance. Using these cloned probes, we shall study the step-wise mechanisms that regulate the synthesis of specific mRNAs by analyzing nuclear and cytoplasmic RNA from embryonic stages that produce these mRNAs and those that do not produce them. Furthermore, sets of mRNAs that are coordinately changed in their abundance during development will be analyzed to determine whether common signals or rate-limiting processes are responsible for their regulation. In addition, we have designed experiments to test the existence of genes that produce only poly(A) minus mRNA or else determine the circumstances wherein genes might produce both poly(A) ion and poly(A) minus mRNAs.