PART I GENERAL 



CHAPTER I 

 MORPHOLOGY AND DEVELOPMENT OF YEASTS 



General Characteristics of Yeasts. The Different Phases in 

 Their Development 



THE yeasts are unicellular fungi; generally, they live in isolated 

 states. After they have acquired a certain size, they divide and 

 produce daughter cells which are not slow to separate, enlarge, 

 and divide in their turn in the same manner. 



In almost all of the yeasts, cellular division takes place by budding 

 or gemmation. There are a few species from warm climates in which 

 multiplication is effected by transverse division. By reason of this 

 particular method of division, these species have been brought together 

 into a special group and named Schizosaccharomycetaceae. 



Budding consists in the appearance, on any side of the cell, of a 

 little bud which slowly increases in size until it ultimately becomes a cell 

 identical with the mother cell. 



Partition simply consists of the formation in the middle of the cell 

 of a wall which divides the cell into two daughter cells of equal size 

 which grow eventually. 



When division of the cell takes place rapidly, it often happens that 

 many buds are formed simultaneously at different points on the sur- 

 face, and these daughter cells begin to multiply before separating. 

 This forms a little colony, or conglomeration, of cells. The same phe- 

 nomenon is observed in the case of the Schizosaccharomycetaceae. 



In old cultures and under certain conditions, the cells remain united 

 in long chains; this gives the appearance of a mycelium, but it always 

 remains in a rudimentary state. 



The yeasts are then able to present two forms: one, which is most 

 frequent, represents the normal type of vegetation. In this the 

 cells are isolated or united in little groups; the other, which is quite 

 exceptional, is the filamentous or mycelial state. 



When the yeast finds, in the medium in which it is cultivated, favor- 

 able conditions for growth, it divides actively until the conditions 

 become unfavorable from the lack of food or the accumulation of 



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