SYSTEMATICALLY SELECTED YEASTS. 75 



is inserted in the neck b. The side-tube is also plugged with 

 cotton-wool, and the flask is then sterilised by heating it for 

 two hours at 150 C. When it has cooled, the tube is 

 joined to the rubber of a two-necked flask in which the yeast 

 has been grown, and a drop of the fairly thick yeast is poured 

 on to the layer of cotton-wool e. The tube is then closed 

 by a stopper d of asbestos- card previously sterilised in a 

 flame, and the stopper is then coated over with a layer of 

 sealing-wax c. 



The main point which I had in view in this arrangement 

 was to preserve the yeast from infection without hermetically 

 closing the flask. The layer of cotton-wool e must therefore 

 be firmly pressed to the bottom, so that it will remain in its 

 place, and for this reason also no more liquid should be 

 poured in with the yeast than is absolutely necessary. 

 Should a little of the liquid, however, run out of the cotton- 

 wool when the flask is turned upside down, it would not 

 escape, as the side-tube is perfectly closed, and it could 

 not pass the neck b, but would be absorbed by the cotton- 

 wool. When, subsequently, the yeast is 

 going to be used, the sealing-wax c 

 must be scraped off, and the side-tube 

 and the whole surface of the flask 

 passed through a flame ; the asbestos 

 stopper is either pushed into the flask 

 or withdrawn from it by means of a 

 pair of forceps, and the tube is then 

 introduced as quickly as possible into 

 the rubber tube of a Pasteur flask con- 

 taining nutrient liquid. If we do not 

 wish to use this flask, the sterile nutrient 

 liquid can very easily be introduced 



through the side-tube by means of a pipette. In the place of 

 the cotton-wool we may also have a layer of gelatine ; but as 

 it is difficult to sterilise this substance without running the 



