W. S. VINCENT 7 



with the absorption of nucleoU which had been fixed in formahn 

 prior to measurement. The RNA was then removed with hot per- 

 chloric acid and the absorption of nucleoh was remeasured. The 

 latter value was taken as the nonspecific absorption and was sub- 

 tracted from the total absorption of the nucleolus; the difference 

 was considered to be due to the RNA present. The results are 

 shown in Fig. 3 in which the upper curve indicates the amount 

 of RNA in formalin fixed nucleoli and the lower curve, the values 



150 



100 



50 



RNA CONTENT of 



INDIVIDUAL NUCLEOLI 



TOTAL 



ISO 



100 



50 



05 10 15 20 " 



DIAMETER of NUCLEOLUS (Micro) 



Fig. 3. RNA content of individual Asterias ruhens nucleoli fixed in 

 formalin (dotted and shaded area) and nucleoli isolated in distilled water 

 (dotted area only). Measurements were made with a microphotometer 

 after Lison (1950) employing Beck reflecting objective and condenser 

 N.A. 0.65, lOX quartz ocular. Light source: 2537 A. line of low pressure 

 mercury lamp. 



obtained from nucleoli isolated in distilled water. The difference 

 between the two curves (shaded in the figure) is of interest. First, 

 it indicates that there is less RNA in isolated nucleoli than in 

 fixed nucleoli of the same size. Second, the pattern of difference 

 in RNA content is proportional to the total amount present until 

 the diameter of the nucleolus reaches about 12 microns. Beyond 

 this, the difiierence between the two remains a constant value. A 

 possible interpretation of this change will be discussed below. 

 The dijfferences in RNA content found in this experiment comple- 

 ment the radiophosphorus studies, suggesting again that starfish 



