ExtractIn order to establish the mode of differentiation in various developing organs, the patterns of nucleic acid synthesis were studied in 14 human fetuses. The gestational age ranged from 8-15 weeks. Incorporation of radioactive precursors into DNA and RNA of cells in liver, kidney, heart, skin, spleen, lung, and brain were measured by autoradiography. Unique patterns of nucleic acid synthesis were observed in each of seven organs studied. The curves of DNA and RNA synthesis were "mirror images" of each other, which indicated organ-specific periodicity of nucleic acid synthesis. Speculation

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