The gathering strength of the messenger hypothesis,' and accumulating evidence in favor of polyribosomes as the sites of protein synthesis,2 are exerting strong influence on current research in chemical embryology. The scheme now generally accepted for microorganisms and certain mammalian cells" 2 provides a number of specific mechanisms by which cellular differentiation could be initiated and controlled. Their essential feature is a direct genomic regulation of the spectrum of proteins made in different cells of the developing organism.Sea urchin eggs, with a long history of use in experimental embryology,3 are a particularly favorable material, because they can be obtained in quantity, develop with excellent synchrony, and are reasonably permeable to labeled precursors. During the past two years, a conflict has arisen from experiments on macromolecule synthesis during early development in these forms. Some of the data were available earlier, but the conflict itself stems from the requirements of the current scheme of protein synthesis, according to which the sequence information is carried by moreor-less unstable messenger RNA's. Protein synthesis is either greatly stimulated or actually switched on at fertilization. RNA synthesis is negligible or absent before fertilization, and even after fertilization, is either very slow4-6 or absent.7 Brachet et al.7 and Gross and Cousineau8 have expressed doubt that the postfertilization synthesis of messenger RNA could be sufficient to account for the observed stimulation of protein synthesis. Nemer5 and Wilt,6 among others, consider that postfertilization RNA synthesis does supply missing templates, and that this gives competence to previously inactive ribosomes and may therefore switch on protein synthesis. The inactivity of ribosome preparations from unfertilized eggs, their activation upon fertilization, and their responsiveness to poly-U9 10, 12, 24 have been used in support of the second hypothesis. Consideration of the behavior of parthenogenetic merogones and experiments with actinomycin D4 11 have, however, led Gross and Cousineau8 to support the idea that templates for the early proteins may pre-exist in the unfertilized egg. Tyler'2 has reported experiments on egg fragments and homogenates thereof whose results are consistent with such an idea.The experiments reported here do not prove that templates pre-exist in the unfertilized egg; but if templates are a requirement for all protein synthesis, they make such a conclusion reasonable. They suggest strongly that the early acceleration of protein synthesis following fertilization cannot depend upon new messenger RNA.

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