BACTERIA AND FERMENTATION 121 



to grow his yeast cells. Well-baked plaster of Paris is 

 mixed with distilled water, and made into a liquid paste. 

 Small moulds are made by pouring this paste into card- 

 board dishes, where it hardens again. The mould is steril- 

 ised by heat, and a small portion of yeast is placed on its 

 upper surface, and then the whole is floated in a small ves- 

 sel of water and covered with a bell-jar. Under these con- 

 ditions of limited pabulum the cell undergoes the following 

 changes : it increases in size, loses much of its granularity, 

 and becomes homogeneous, and about thirty hours after 



Gypsum Block 



being sown on the gypsum there appear several refractile 

 cells inside the parent cell. These are the ascospores. In 

 addition to the gypsum, it is necessary to have a plentiful 

 supply of oxygen, some moisture (gained from the vessel of 

 water in which the gypsum floats), a certain temperature, 

 and a young condition of the protoplasm of the parent yeast 

 cells. Hansen found that the lowest temperature at which 

 these ascospores were produced was .5-3° C, and at the 

 other extreme up to n"" C, which is blood-heat. The 

 rapidity of formation also varies with the temperature, the 

 favourable degree of warmth being about 22-25° C. 



Hansen pointed out that it was possible by means of 

 sporulation to differentiate species of yeasts. For it hap- 

 pens that different species show slight differences in spore 

 formation, e. g. : 



