GENETICS AND CYTOLOGY OF SACCHAROMYCES 261 



red to white resembles the discontinuous variations which occur in the vege- 

 tative cycles of bacteria. Hybridization experiments have revealed that the 

 origin of white cultures does not involve gene change. This phenomenon in 

 yeast, called depletion mutation, is identical with Dauermodifikation in Para- 

 mecia. Since neither involves gene change, both are equivalent to differentia- 

 tion. 



It is not possible to study Dauermodifikationen using the classical objects 

 of genetical research, maize and Drosophila, since each generation of these 

 higher organisms is produced sexually — a process during which Dauermodi- 

 fikationen revert to normal. The stable variants in vegetative cultures of 

 yeast, which revert to normal (produce only normal offspring by sexual re- 

 production) have no parallel in maize and Drosophila. This points up a 

 striking disadvantage of maize and Drosophila — that they cannot be propa- 

 gated vegetatively. One cannot be certain that the characteristic variations 

 in flies, which occur when they hatch on wet medium or are subjected to 

 shock treatment, would be lost on vegetative cultures unless one were able 

 to propagate the bent wings or other peculiarities asexually, possibly in tis- 

 sue culture. 



THE AUTONOMOUS ORGANELLES OF THE YEAST CELL 



In addition to the chromosomes (Lindegren, 1949) there are other perma- 

 nent structures in the yeast cell which never originate de novo (Lindegren, 

 1951). They have the same type of continuity in time as chromosomes but 

 are less precisely partitioned than the latter. 



The Cytoplasm 



The cytoplasm is a limpid fluid which is transmitted to each daughter cell. 

 It is rich in RNA but varies in basophily and contains the mitochondria, 

 usually adhering to the surface of the centrosome or the nuclear vacuole. 



The Mitochondria 

 The state of the mitochondria varies from highly refractile lipoidal struc- 

 tures, sharply defined from the cytoplasm to less refractile organelles with 

 somewhat irregular boundaries. 



The Centrosome 



The centrosome is a solid and rigid structure which stains with acid 

 fuchsin but does not stain with basic dyes. This highly basic organelle may 

 contain some of the basic proteins which Caspersson and Mirsky have found 

 in chromosomes. The centrosome is always attached to the nuclear vacuole 

 and is the most rigid structure in the cell as revealed by its behavior following 

 shrinkage of the cell. It never originates de novo and plays a leading part in 

 budding, copulation, and meiosis. 



