44 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



cells. The young cell has a very thin cell wall, protoplasm 

 which is relatively homogenous, and a relatively large 

 nucleus. The latter is easily demonstrated by the use of 

 appropriate stains. As the cells increase in size, vacuoles 

 begin to appear in the protoplasm ; both in these vacuoles 

 and in the protoplasm are formed granules which stain 

 intensely with methylene blue, the so-called metachromatic 

 granules. Eventually still larger vacuoles develop, which 

 in some eases are crowded full of granules giving the yel- 

 ■"ow glycogen reaction with iodine. Still later the glycogen 

 may disappear but the relatively large vacuoles persist. In 

 an old cell the cytoplasm and nucleus may constitute a 

 relatively small proportion of the cell contents. 



By means of suitable reagents it is possible to demon- 



FlG. 30. — Yeast cells. A. Vegetative cell. n. Nucleus, v. Vacuole. 

 B. Yeast cell filled with glycogen. 



strate the presence of an outer layer of the protoplasm, the 

 ectoplast, corresponding in function to that found within 

 the bacteria. It undoubtedly acts as a semipermeable 

 membrane in the determination of substances which may 

 leave and enter the cell. 



Vegetative Multiplication of Yeast Cells. — Most of the 

 common yeasts multiply vegetatively by a process of bud- 

 ding. A few species belonging to a single genus {Schizo- 

 saccharomyces) multiply by fission, in a manner analogous 

 to that already described for bacteria. 



Those yeasts which multiply by budding show first a 

 minute protuberance on one side of the cell. The nucleus 

 of the mother cell goes to a point near the opening between 



