ALCOHOLIC FERMENTS. 143 



With the help of the acquired knowledge of the species it 

 was possible to submit these methods to a searching examina- 

 tion, with the result that they proved to be reliable. 



If it is desired to isolate from a mixed growth of different 

 species those which are in an enfeebled condition, it is necessary, 

 as HANSEN points out, to employ the dilution method, using a 

 suitable nutrient fluid, such as wort, the conditions being then 

 favourable for the growth of the organisms in question. The 

 yeast is introduced into flasks containing the liquid, or into 

 drops, each of which is kept in a moist chamber. 



If, on the other hand, we wish to separate from a mixed 

 growth a species which is in a vigorous state of development, 

 and whose further growth is consequently not dependent upon 

 specially favourable conditions of nutriment, we can attain our 

 object more readily and in a shorter time by the employment 

 of a solid nutrient medium in this case gelatine and wort. 



It has been proved that the addition of gelatine to wort 

 diminishes its value as a nutritive material for the yeast- 

 fungi. A series of experiments carried out by HOLM show in 

 fact that, if some of these cells are introduced into wort- 

 gelatine at the commencement of a fermentation, when the 

 yeast-cells are in their most vigorous state of development, 

 about 4 per cent, of those sown do not develop ; if, on the 

 other hand, the yeast-cells are taken at the conclusion of a 

 fermentation, when they are enfeebled, about 25 per cent, give 

 no colonies in wort-gelatine. 



The advantage of this method, as employed by HANSEN, 

 for the study of budding-fungi is that it makes it possible to 

 directly observe individual cells under the microscope and to 

 follow their further development, for the gelatine plate or the 

 drop is enclosed in a moist chamber (compare Chapter I., 

 " Dilution Methods "). 



2. THE ANALYSIS. 



Throughout the entire series of HANSEN'S researches a 

 leading idea obtains, namely, that the shape, relative size, and 

 appearance of the cells, taken ~by themselves, are not sufficient to 

 characterise a species, since the same species, when exposed to 



