62 



MORPHOLOGY AND CULTURE 01" MICROORGANISMS 



In order to understand dearly this structure, one must observe 

 young cells taken from a culture at the beginning of development. 

 For this purpose we use Sacchafomyces cerevisia which, because of the 

 relatively large size of its cells, lends itseK better than 

 any other yeast to a cytological study. Examined in 

 the hving state, highly magnified, the cells of this 

 yeast show a dense and homogeneous cytoplasm with 

 a group of small vacuoles or a single large vacuole at 

 the center. In the vacuoles and also in the perivacu- 

 olar cytoplasm, we can clearly distinguish a great 

 many small shining granules, of varying sizes, which 

 manifest Brownian motion. It is easy to stain them 

 in the living state (Fig. 43) with a very dilute solu- 

 tion of neutral red or methylene blue. These are 

 only metachromatic corpuscles. 



In fixed and stained preparations (Fig. 44, i-io) is 

 seen in each cell a single, large nucleus, whose struc- 

 ture is exactly like that which we have discussed in molds. This 

 nudeus is surrounded by a membraine arid contains a hyaline nucleo- 



FiG. 43. — Sac- 

 charomyces cere- 

 visicB. Young 

 cells examined in 

 the living state 

 in a solution of 

 neutral red. The 

 vacuoles, stained 

 pale red, contain 

 metachromatic 

 corpuscles col- 

 ored dark red. 



Fig. 44. Fig. 45. 



Fig. 44. — Saccharomyces cerevisia. i-io, Young cells with nucleus, showing its 

 structure. 6-8, The same: division of the nucleus. 11-13, Cells after twenty-four 

 hours' fermentation, with a very large glycogenic vacuole filled with lightly colored 

 grains. 



Fig. 4S- — Saccharomyces cerevisid. Young cells fixed and stained by a special 

 method revealing in the cytoplasm a chondrium consisting of rod mitochondria and 

 granular mitochondria. 



plasm in which is easily seen a large nucleolus and some chromatin; 

 this latter is scattered through the nucleus, sometimes found in the 

 nudeoplasm in the form of a network, sometimes reduced to a num- 



