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Fig. 2. (rt) Autoradiograph of a Tetra/iytneiia incubated in [^H]-cytidine for i -5 

 min. All incorporation is localized in the nucleus. 



(b) Autoradiograph of a Tetrahytnena incubated in [-'HJ-cytidine for 12 min. 

 .All label is still localized in the nucleus. 



(c) Autoradiograph of a Tetrahytnena incubated in [^H]-cytidine for 35 min. 

 Nucleus and cytoplasm contain equal amounts of label, although the nuclear label 

 is still more dense. 



(d) Autoradiograph of a Tetrahymeiia incubated in [^H]-cytidine for 12 min. 

 followed by incubation in non-radioactive medium for 88 min. The cytoplasmic 

 RNA is heavily labelled, but the nucleus contains no labelled RNA. 



VOL. ir. — 2 M 



