Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

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More About Levels of Control. If sea urchin zygotes are exposed to actinomycin D, an inhibitor of RNA synthesis, during the early stages of embryonic development, protein synthesis and development continue until the embryos form a hollow ball of cells, or blastula, with hundreds of cells. However, if the same inhibitor is added later during embryonic development, protein synthesis is severely depressed and normal embryonic development halts. How might you explain these observations