6-27 



THE YEAST CELL 



than in an older one. The chromosomes in the nuclear vacuole are 

 practically always paired after the cells have been growing for six 

 hours or more. There appear to be four or five pairs in S. bayanus 

 and five or six in S. cerevisiae, Mrak 77. They appear to have the 

 shape of paired cylindrical rods or crescent-shaped coffee-bean 

 like structures. They stain a dark green with methyl green and a 

 deep red with Feulgen following chrome -acetic -formaldehyde fixa- 

 tion. Methyl green is an acceptable chromatin stain and has long 

 been recognized for its affinity for chromosomes. When chrome- 

 acetic -formaldehyde is followed by acid fuchsin-methyl green the 

 centrosome always appears as a dense red body surrounded by an 

 area which is pale pink. In these preparations the nuclear mem- 

 brane can be traced to include all the centrosome. The nucleolus is 

 is pale green and is always visible after l| hours incubation. Both 

 nucleolus and chromosomes are not generally visible simultane- 

 ously. The pronoimced and constant distinction between the cen- 

 trosome and the chromosomes suggests that these structures which 

 are both Feulgen-positive are different in some other respect than 

 in content of desoxyribose-nucleoprotein. 



Fig. 6-21 A Feulgen-stained (using Rafalko's modification) 

 yeast ceil in a preparation made by Dr. Rafalko showing four pairs 

 of Feulgen-positive chromosomes in the nuclear vacuole, one pair of 

 which is attached to the Feulgen-negat ive nucleolus. 



The methyl green-pyronin combination aiwa^/^ shows the cen- 

 trosome as a pale green structure. The chromosomes are dark 

 green when visible and the nucleolus is pale green when visible. 

 Either chromosomes o r nucleolus are usually visible along with 

 the constantly appearing centrosome. Sometimes all three are 



