122 BACTERIA 



(a) The spores of Sacchanomyces cerevisim expand during 

 the first stage of germination, and produce partition walls, 

 making a compound cell with several chambers. Budding 

 can occur at any point on the surface of the swollen spores. 

 To this group belong S, pastorianus and 5". ellipsoideus, 



{b) The spores of Saccharomyces Liidwigii fuse in the first 

 stage, and afterwards grow out into a promycelium, which 

 produces yeast cells. 



{c) The spores of Saccharomyces aiioinalus are different in 

 shape from the others in that they possess a projecting rim 

 round the base. 



Another point in the cultivation of yeasts has been eluci- 

 dated by a number of workers, chief among whom perhaps 

 is Hansen, namely, methods of oh\.2Xx{\xi^ pure cultures. We 

 know, generally speaking, what this term means, and there 

 is no difference in its meaning here to what is understood as 

 its meaning with regard to bacteria. There is, however, 

 some difference in the mode of securing it. It is only by 

 starting with one individual cell that we can hope to secure 

 a pure culture of yeasts. For the study of the morphology 

 of yeasts under the microscope the problem was not a diffi- 

 cult one. It was comparatively easy to keep out foreign 

 germs from a cover-glass preparation enough to perceive 

 germination of spores and growth of mycelium. But when 

 we require pure cultures for various physiological purposes, 

 then a different standard and method are necessary. 



Pasteur and Cohn adopted a practice based upon the fact 

 that when organisms find themselves in a favourable medium 

 they multiply to the exclusion of others to which the medium 

 is less favourable. Hence if an impure mixture be placed 

 under such circumstances there comes a time when those 

 organisms for which the circumstances are favourable mul- 

 tiply to such an extent that they form an almost pure cult- 

 ure. The method is open to fallacy, and will rarely result 

 in a really pure culture; and even if that be secured, it is 



