FERMENTATION 465 



Fermentation 



The yeasts are of great importance in inducing many chemical 

 changes, especially alcoholic fermentation, beer and wine being 

 almost exclusively due to their activity. 



Taking brewer's yeast, Saccharomyces cerevisice, as a type, the 

 yeast cell is observed to be slightly ovoid in shape, measuring 8 to 

 9 p. in diameter. The protoplasm is granular, contains one or more 

 clear spaces or vacuoles, frequently bright, refractile globules of 

 fatty matter, and is surrounded by a cell wall of cellulose. It has 

 been repeatedly stated that a nucleus is present, but this is doubtful. 

 When the yeast-cell is freely supplied with nutriment, reproduction 

 by gemmation proceeds rapidly, and a whole string of cells may 

 form owing to the daughter-cells budding again before they have 

 separated from the parent. When the cell is starved, gemmation 

 ceases, fat-globules and vacuoles increase in number, and the cell 

 may finally become little more than a large vacuole, the protoplasm 

 forming a thin coating over the inside of the cell wall. Within the 

 vacuoles are often seen minute spherical bodies of a doubtful nature 

 in rapid movement. In ordinary circumstances endospore forma- 

 tion does not occur, but by deprivation of nutriment, as by growing 

 on a block of plaster-of -Paris, the cells develop spores. First the 

 cell becomes divided by the development of membranes, the so- 

 called " partition- wall formation," into several chambers in which 

 the spores form. In the different yeasts the number and arrange- 

 ment of the spores vary ; in the S. cerevisice the typical number is 

 four, arranged close together, three on one plane and one resting 

 on these, like a pyramid of billiard balls. 



Although the reproduction of yeasts by gemmation or ascospore 

 formation is usually asexual, ascospore formation is sometimes 

 preceded by conjugation of sister-cells, or conjugation may occur 

 between neighbouring cells at the moment of germination (Guillier- 

 mond, Nadson, and Marchand). 



The spores are of considerable importance in the identification 

 of species of Saccharomyces, as the form of the cells alone and the 

 growths on culture media are not sufficiently distinctive. In fact 

 so little can these two characters be relied upon that in order to 

 isolate in pure cultivation it is necessary to grow from a single cell. 

 This can be done by making a miniature plate cultivation with 

 wort-gelatin on a large sterilised cover-glass, and, after the layer of 

 gelatin has set, mounting, gelatin downwards, on a large cell on a 

 glass slide. The cover-glass should be divided into small squares 



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