1. The significance of the term nucleolus has been discussed.

2. A detailed method for the isolation of nucleoli from already isolated rat or cat liver nuclei has been presented.

3. The presence of DNA in isolated liver cell nucleoli has been indicated by histochemical methods.

4. The percentages of DNA and RNA in the isolated nucleoli have been determined by chemical analysis.

5. The specific activities of aldolase, arginase, and catalase have been determined for two subnuclear fractions and for the isolated nucleoli of rat and cat liver, and the relative amounts of these enzymes in the same subnuclear fractions and nucleoli of rat liver have been measured.

6. The significance of the above findings has been discussed and consideration has been given to what types of isolated nuclei might best serve as starting material for the isolation of nucleoli.

7. A new hypothesis has been presented that nucleoli of the liver cell type may function primarily in furnishing (directly or indirectly) templates for the synthesis of the particular enzymes that must govern the chemistry of mitosis.

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