FERMENTATION 563 



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 move- 

 ment. In ordinary circumstances endospore formation 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 8. 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 (Guil- 

 liermond, 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 by cross-lines etched on the glass and numbered. 

 The preparation is carefully examined with a or i-in. objective, 

 and the positions of single isolated cells are noted. This is not a 

 difficult matter on account of the comparatively large size of the 

 yeast-cells, and their position is determined by the numbered 

 squares on the cover-glass. The preparations are kept in a 

 moist chamber in. a warm place, and when visible colonies have 

 developed, those which are derived from a single cell can be 

 inoculated into tubes or flasks of a suitable culture medium. 



It is found that the various yeasts form spores in different 

 periods of time when grown under similar conditions, and on this 

 fact is based what is known as the analysis of yeast a most 

 valuable method, which we owe to Hansen. The chief " diseases " 



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