FERMENTATION. IO3 



methods for obtaining pure cultivations, were right. These 

 observers had ascribed to certain forms of yeasts the power 

 of forming spores ; whilst in others the same power was 

 denied, Wiesner holding that spores could not develop from 

 pressed yeast, although the^ could from beer yeast, and 

 Brefeld maintaining that cultivated yeast had lost its power 

 of forming spores, whilst the wild yeast still retained this 

 faculty. To evolve some kind of order from this confusion 

 was the task that Hansen set himself ; he wished to determine 

 the conditions under which ascospores could be formed. 



For this purpose he used the method of complete aeration that is obtained 

 by the use of Engel's gypsum blocks. To well-baked plaster of Paris add 

 distilled water until the plaster is nearly liquid ; pour this on to a sterilized 

 glass plate, on which rests a small mould of thin metal or paper, made rather 

 less than the vessel in which the experiment is to be carried on. These 

 blocks are first thoroughly sterilized by means of heat, after which a small 

 particle of yeast is placed on the upper surface of one of them, an air 

 chamber is sterilized, and in this a small vessel containing water, in 

 which the block rests, is kept. The whole vessel may be placed in an 

 incubator if any special temperature is required, or it may be left at the 

 ordinary temperature of the room. 



He found that the following conditicwis were necessary for 

 the perfect formation of spores : a plentiful supply of air 

 (oxygen) and moisture ; a certain temperature — the most 

 suitable for the six forms that he examined being about 25° 

 C. ; a young condition of the protoplasm of the yeast-cells, 

 the older cells with their thickened walls appearing seldom, 

 if ever, to give rise to spores. As regards temperature, he 

 found that the extremes, at which the individuals of the 

 different species grow, vary somewhat, the lowest tempera- 

 ture at which they are developed being from .5° to 3° C. ; 

 whilst at the other extreme he found that they could still 

 grow at a temperature of 37.5° C He found that spores 

 became visible as irregular bodies formed from the cell 

 contents in a period of about thirty hours from the com- 

 mencement of the sowing of the yeast-cells when the 

 temperature was kept near the higher extreme, or as low 

 down as 25° C. ; but working with lower temperatures, 

 differences occur in the different species, though in none of 

 them does the development of spores take place so rapidly 

 as at the higher temperatures ; for example, he found that at 

 11.5° C. the Saccharomyces cerevisise does not form spores 

 until a period of ten days has elapsed ; whilst, on the other 



