WHAT IS THE PURE YEAST OF PASTEUR? 131 



when other yeasts or mould fungi are present together with 

 Saccharomyces apiculatus. In such cases it is best to abandon 

 the experiment at once and to start again from the be- 

 ginning. If a satisfactory culture has been obtained in the 

 manner described it is made use of to inoculate a two-necked 

 Pasteur flask containing sterilised wort to which a little 

 tartaric acid has been added. After a few days, when fer- 

 mentation is in progress, the nutrient liquid is decanted from 

 the yeast which has settled on the bottom of the flask, and 

 fresh liquid of the same composition is added, the operation 

 being performed with proper precaution so that external 

 organisms do not enter. When this process has been repeated 

 a few times a perfectly pure culture can, at last, be obtained." 

 Notwithstanding the insufficiency of the method, I was able 

 to express myself thus definitely, because the species in 

 question possesses, in several respects, very marked character- 

 istics. It is, indeed, one of the few species of yeast which, 

 owing to the peculiar form of its cells, we are able to recognise 

 by a simple microscopic examination, and, in addition to 

 this, in its physiological aspect it also presents several peculi- 

 arities. In working with this species it is, indeed, possible at 

 every stage of the experiment to prove whether we are 

 dealing with a pure culture or not. This does not hold good 

 for yeast cells with endogenous spore formation, i. e. the true 

 Saccharomycetes* and it was just these which I wished to 

 submit to a thorough investigation on account of their great 

 theoretical and practical importance. In this field it was not 

 possible to proceed further along the paths marked out by 

 my predecessors. In order that I might be in a position to 

 attack the problems I had set myself, it was, therefore, 

 necessary in the first place to elaborate an exact method for 

 the preparation of the necessary pure cultures. I was thus 

 compelled against my will to devote some years to the study 

 of this branch of bacteriological technique, and it was only 



* A few recently discovered species form the only exceptions. 



K 2 



