150 ALEXANDER L. DOUNCE 



follows from the fact that if total nucleic acid is measured (after extraction) 

 by absorption spectroscopy, and if DNA is then measured by Schneider's 

 adaptation of the Dische technique, the results agree within a few per cent, 

 leaving only 2 to 4% of the material which could be PNA. Moreover, as 

 has been stated, it has been found that isolated rat liver nucleoli are defi- 

 nitely Feulgen-positive. An additional observation is that some of them 

 stain purple with the methyl green-pyronin stain, some stain pink, and 

 some are intermediate in color, although no differences in morphology have 

 yet been observed which might correlate with this observed variation in 

 staining. 



All these findings could be explained by assuming the presence of a rather 

 low polymer type of DNA in the liver cell nucleoli. This assumption would 

 explain the variable staining with methyl green-pyronin and the positive 

 Feulgen stain, as well as the gross chemical findings. Moreover, it is possible 

 that a low-polymer DNA might be changed by fixatives or even be partly 

 lost during fixation. If this assumption should eventually be proved valid, 

 it would be necessary to reinvestigate nucleoli of cells other than those of 

 liver, to be sure that no artifacts have occurred. However, a second possible 

 explanation of the occurrence of DNA in rat liver nucleoli is adsorption by 

 the nucleoli of perinucleolar material rich in DNA. 



This Feulgen-positive perinucleolar material which can be observed en- 

 circling the nucleoli of certain mammalian cell nuclei deserves special 

 mention. The perinucleolar ring apparently is not an artifact of fixation as 

 might be surmised, but is real, since Austin^**" has demonstrated that a 

 ring showing the same morphology as the Feulgen-positive ring in rat egg 

 cell nucleoli can be observed using the phase microscope with living egg cells. 

 An identical ring can also be photographed with ultraviolet light. Such 

 perinucleolar material cannot be observed in nucleoli isolated in the writer's 

 laboratory, but it is possible that the ring might collapse on the nucleoli 

 during the isolation procedure so as to form a thin, microscopically invisible, 

 surface layer rich in DNA. This is a second possible explanation for the 

 high DNA content of the isolated nucleoli. 



It may therefore turn out that the Warren Lewis as well as the Caspers- 

 son-Schultz type of nucleoli do not contain DNA within their structure. 

 It is suggested that the nonspecialist should keep a rather open mind on the 

 problem of the composition of mammalian nucleoli with respect to DNA 

 until more results have accumulated, but it may be asking too much of the 

 specialist to suggest that he do the same. 



Finally, the possible function of the nucleoli in the cell may be briefly 

 discussed. Caspersson and co-workers^*^ believe that the nucleolus is con- 

 cerned with synthesis of ribonucleic acid for the cytoplasm. The observa- 



^oo C. R. Austin, Exptl. Cell. Research 4, 249 (1953). 



