6-39 THE YEAST CELL 



tinually increases in spite of the fact that volutin first appears in 

 the chromosomes and then disappears from them and continues to 

 accumulate later exclusively in the cytoplasm. Concurrent stains 

 with the methylene blue dead cell stain reveal that the cells con- 

 taining volutin in the cytoplasm are dead. This is consistent with 

 the fact that metaphosphate is capable of precipitating proteins. The 

 appearance of volutin in detectable amounts in the cytoplasm is a 

 definite criterion of death. The rate of death can be reduced by 

 adding the phosphate to the stained cells very slowly, but in a medi- 

 um containing only 2 per cent sugar the phosphate eventually results 

 in the death of the cells. When normal cells (in an adequate medium) 

 take up phosphate, the volutin stain reveals that as growth progresses 

 the cytoplasm becomes blue and the chromosomes red, indicating 

 that there is protein but no stainable volutin in the cytoplasm of the 

 healthy yeast cell. Healthy growth only occurs if a properly bal- 

 anced nutrient is added slowly; phosphate alone produces a condi- 

 tion of imbalance which eventually results in death. 



VOLUTIN AND RIBOSENUCLEOPROTEIN SYNTHESIS 



The loss of stain from the chromosomes and its accumulation 

 in the nucleolus indicates that the volutin formed on the chromo- 

 somes has been transferred to the nucleolus. Apparently, the syn- 

 thesis of volutin occurs on the surface of the chromosomes, and as 

 soon as the chromosomes becomes densely loaded, the transfer to 

 the nucleolus occurs. Caspersson and Schultz have described the 

 nucleolus of higher plants and animals as the center for the syn- 

 thesis of ribosenucleoprotein. They showed that after ribosenucleo- 

 protein is synthesized in the nucleolus in the cell of a higher plant 

 or animal, it travels through the membrane into the cytoplasm. 

 Caspersson and Brandt did not observe the nucleolus in the yeast 

 cell with ultra violet light, possibly because the cytoplasm contains 

 so much nucleoprotein that the smaller amount in the nucleolus is 

 overshadowed. They did show (with ultra violet light) that ribosenu- 

 cleoprotein accumulates on the outside of the nuclear vacuole. They 

 thought it was synthesized at this point, but the demonstration of a 

 conventional nucleolus in the nuclear vacuole suggests that the ac- 

 cumulation of ribosenucleoprotein, demonstrated on the surface of 

 the nuclear membrane, may be the synthesis of this substance, due 

 to the diffusion of ribose nucleic acid from the nucleolus through 

 the nuclear membrane and into the cytoplasm. This view is in gen- 

 eral agreement with Caspersson and Schultz' s ideas concerning 

 the role of the nucleolus in the synthesis of ribosenucleoprotein 

 in higher plants and animals, but Caspersson and Brandt's find- 

 ings concerning the yeast cell were based on the incomplete cyto- 

 logical information on the yeast cell available at that time. 



