CYTOLOGV 6-32 



solution. Figs. 6-23 and 24 are preparations of yeast cells stained 

 with Lugol's and reveal nucleoli in many of the vacuoles. The cen- 

 trosomes are also visible in many cells] If the chromosomes are 

 coated with volutin, they are stained purple (fig. 6-25). If the nu- 

 cleolus is coated with volutin, it is stained purple by the volutin 

 stain (fig. 6-26). As soon as either the chromosomes or the nu- 

 cleolus lose their volutin, they are no longer specifically stain- 

 able. The density of the stain also gives some idea of the quan- 

 tity of volutin present. The best results are obtained when the 

 volutin stain is adjusted to a pH of 2.5, which prevents the pro- 

 teins in the cytoplasm from also becoming stained. 



The growth of S. cerevisiae was followed closely from resting 

 to actively-growing, vigorously -fermenting cells (fig. 6-28). In 

 resting cells, the chromosomes are unstained and imstainable by 

 either the volutin stain or iodine, and the nucleolus is invisible or 

 absent. After the cells have been in nutrient medium for a short 

 time, (before volutin appears on the chromosomes) the chromo- 

 somes stain brown with iodine suggesting that they may contain 

 carbohydrate. Fig. 6-27 is a photograph of cells fixed in dilute 

 formalin and stained with iodine. Before budding begins, the chro- 

 mosomes become coated with volutin, and two unstained nucleoli 

 become visible in each cell. As budding begins, the nucleoli in- 

 crease in size, volutin disappears from the chromosomes and ap- 

 pears on the nucleolus. Transfer of volutin occurs by a direct at- 

 tachment of chromosome to nucleolus , for two or three small, 

 dark purple dots can be seen attached to an unstained nucleolus. 

 Generally, one nucleolus becomes coated with volutin before the 

 other, and subsequently both fuse. These fused nucleoli are the 

 commonly described refractile "dancing bodies" in the yeast vacu- 

 ole. The nucleolus is a large, round structure, possibly 2 or 3 

 microns in diameter. At first the interior of the nucleolus is not 

 deeply stained with the purple dye; the color is on the outer edge 

 or surface of the structure. The nucleolus often becomes quite 

 large before volutin appears on it. 



The following typical results were obtained when 7 grams of 

 compressed yeast were inoculated into 500 cc. of medium of the 

 following composition: 



The culture was grown in a 6-liter Erlenmeyer flask on a shaker. 

 Yeast cells in a cake of baker's yeast are small and compact 

 and ordinarily show little or no volutin in the vacuole. During the 



