124 MICRO-ORGANISMS AND FERMENTATION. 



2. THE ANALYSIS. 



Throughout the entire series of Hansen's researches a 

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

 and the appearance of the cells, taken by themselves, are not 

 sufficient to characterise a species, since the same species, 

 when exposed to different external conditions, can occur in 

 very different forms and quite different in appearance. On 

 the other hand, the forms of development of the cells, 

 regarded from another point of view, constitute very impor- 

 tant distinctive characters for different species. Thus it is 

 found that different species under the same treatment behave 

 differently and assume different forms. This can only be 

 explained by assuming that there are intrinsic, indwelling 

 characters in the special cells which exert an influence of 

 their own. 



In the following we give a brief account of the various 

 means by which Hansen determined the characteristics of 

 different species. These investigations form at the same time 

 contributions to the general physiology of the budding-fungi. 



(a) The Microscopic Appearance of the Sedimentary 

 Yeast. The first examination of a yeast will generally consist 

 in observing under the microscope the appearance of the sedi- 

 ment. As examples illustrating what can be ascertained in 

 this way, we may call attention to the following figures (Figs. 

 34, 37, 39, 41, 43, 45), representing the young sedimentary 

 forms of the six species of Saccharomycetes which have been 

 specially investigated by Hansen. The growths were obtained 

 by cultivating the cells for some time in wort, then intro- 

 ducing fresh wort, and by maintaining a temperature of 25 

 to 27 C. for 24 hours a vigorous growth was developed. If 

 we now compare, for instance, the figures representing Sac- 

 char omyces cerevisice I., with those which illustrate the three 

 Pastorianus species, we find that, taken as a whole, they show 

 marked differences. Saccharomyces cerevisice consists mainly 

 of large round or oval cells, the Pastorianus species form 



