318 



CELL HEREDITY 



In larvae of Drosophila rcplcta. the cells of the salivary gland grow 

 rapidlv during the last few days before pupa formation. During the 

 period of the experiment to be described, the nuclear and cytoplasmic 

 volumes tripled, and the nucleolus doubled its volume and RNA content. 

 Larvae selected for uniformity were fed for two hours with adenine-C ''*, 

 and then removed to a nonradioactive food source, prior to fixation at 

 various times. The specific activities of RNA in nucleolus, chromatin, 

 and cytoplasm were determined by grain-counting on autoradiographs of 

 sectioned material. The results indicate (Figure 11.6) that RNA syn- 

 thesis, as measured by the uptake of adenine, proceeds faster in the 

 nucleolus than in any other fraction and, consequently, that neither 

 chromosomal nor cytoplasmic RNA can be the source of all nucleolar 

 RNA. No other information about transfer of label between chromo- 



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^''**>'1t^ 



11 -7a 



FIGURE 1 1 .7. Distal end of salivary chromosome C of Rhynchosciara angelae showing 

 localization of DNA by means of autoradiography of tritiated thymidine incorporated 

 into DNA in comparison with Feulgen staining. 



(o). The larva was injected with tritiated thymidine 24 hours before being killed. 

 The close correlation between bands of DNA which stain intensely with Feulgen reagent 



