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bility may be a reflection of its capacity to stop the synthesis of new ribosomal subunits.10 Thus, the newly transcribed messages may ordinarily be stabilized by association with newly synthesized ribosomal subunits before migration through the nuclear envelope and may be rendered labile to nucleolytic attack when this association is prevented. Summary.-During morphogenesis in Dictyostelium discoideum strain FR-17 there is a 4-hr period of RNA synthesis, delineated by actinomycin sensitivity, which is required for the appearance of the enzyme UDP-Galactose polysaccharide transferase. Interference with protein synthesis during this transcription disrupts the transcriptive process but does not affect the transferase-specific RNA previously fabricated. The cells appear to compensate for the disruption by initiating a second round of transcription after the resumption of protein synthesis. By these means the period of transferase-specific RNA synthesis can be extended considerably beyond its normally prescribed time limits. * This work was supported by a grant from the National Science Foundation (GB-13 10).

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