250 SACCHAROMYCETES. 



species which ordinarily constitute commercial yeast, and to which 

 the name of Saccharomyces was originally applied, are alone in 

 reproducing themselves by spores formed in this particular way. 

 This fact, being fully confirmed, on further investigation, by Hansen 

 himself and others, constitutes a definite test, which enables the 

 genus to be defined strictly as an alcoholic ferment reproducing 

 itself both by sproufi?ig and by means of ascospores. From this 

 stage in the investigation Dr. Hansen was enabled to build up, 

 with wonderful patience and skill, by means of investigations 

 lasting over many years, the system of classification now employed. 



As far back as 1878 the writer, being then engaged in technical 

 research, became convinced that the apparently similar cells in 

 yeast were by no means identical ; in fact, that some fundamental 

 difference existed between different samples, and probably between 

 the cells constituting each. The proof of this was found in the 

 fact that whatever care might be exercised in the microscopic 

 selection of yeast, the result of the action of different samples, 

 in the same nutritive medium, exhibited wide analytical and prac- 

 tical differences. That is to say, when two different yeasts, between 

 which no difference could be observed on most thorough micro- 

 scopic examination, were cultivated in sterilised malt wort under 

 precisely similar conditions, the resulting alcoholic fluids frequently 

 yielded altogether different analytical results. Want of time, and 

 the necessity of carrying on daily technical work, prevented the 

 following up of research thus suggested, but soon after Hansen, 

 who was then engaged on the subject, took it up at this point, and 

 starting from the ascospore formation discovered by Rees, he 

 applied it with unlooked for success as a means of identifying and 

 separating the varieties of cells in commercial yeast. 



It was, at an early point, evident that to obtain results 

 of any value he must work with pure cultures ; that is to say, 

 yeasts which had been grown under such stringent conditions as 

 should insure the presence of one species only. Such a culture 

 could only be obtained by the selection of a single cell and allow- 

 ing it to multiply, with proper precautions against accidental con- 

 tamination, until a sufficient bulk was obtained for experimental 

 purposes. The great difficulty to be overcome was the introduc- 

 tion of a single cell, and no more, into the flask in which the 



