126 MICRO-ORGANISMS AND FERMENTATION. 



1 . The cells must be placed on a moist surface and have 



a plentiful supply of air. 



2. Only young, vigorous cells can exercise this function. 



3. The most favourable temperature for most of the 



species as yet examined is about 25 C. This 

 temperature favours spore-formation in all known 

 species. 



4. A few Saccharomycetes likewise form spores when they 



are present in fermenting nutrient fluids. 

 A growth of yeast is developed in the manner described 

 on page 124. A small quantity is transferred to a previously 

 sterilised gypsum block ; this block is enclosed in a flat 



FIG. 26. 



The first stages of development of the spores of Sacch. cerevisiae I., after 

 Hansen : a, b, c, d, e, rudiments of spores, where the walls are not yet 

 distinct ; /, g, h, i, j, completely-developed spores with distinct walls. 



covered glass and is maintained moist by half filling the 

 glass with water. 1 If it be desired merely to bring about 

 the formation of spores, the apparatus may be allowed to 

 remain at the ordinary room-temperature. 



Hansen was the first to give an accurate description of the 

 structure of spores and a detailed account of their evolution, 

 founded upon observations of individual spores; and he 

 distinguished three typically different groups of Saccharo- 



1 Ascospores can also be obtained when yeast is spread upon sterilised 

 solidified gelatine, prepared with or without a nutrient solution, kept in 

 a damp place ; likewise in yeast- water and in sterilised water ; finally, 

 spore-forming cells also occur in the films of the Saccharomycetes. The 

 method is evidently not dependent upon these different substrata but upon 

 the knowledge of the factors which render it possible for the cells to 

 exercise this function of forming spores. 



