144 MICRO-ORGANISMS AND FERMENTATION. 



different external conditions, can occur in very varied forms 

 and may differ greatly in appearance. On the other hand, 

 the forms of development of the cells, regarded from another 

 point of view, constitute very important 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, natural characters in the special cells which exert 

 an influence of their own. 



There follows a brief account of the various means by 

 which HANSEN determined the characteristics of different 

 species. These investigations form at the same time con- 

 tributions 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 the appearance of the sediment under the micro- 

 scope. As examples illustrating what information may be 

 gained in this way, we call attention to the figures 47, 50, 

 52, 54, 56, 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 introducing 

 fresh wort, and maintaining a temperature of 25 to 27 C. for 

 24 hours ; thus a vigorous growth was' developed. Comparing, 

 for instance, the figures representing Saccharomyces cerevisice I. 

 with those which illustrate the three Pastorianus species, we 

 find that, taken as a whole, they show marked differences. Sac- 

 charomyces cerevisice consists mainly of large round or oval 

 cells, the Pastorianus species form mostly elongated sausage- 

 shaped cells. It is, however, a very different matter when 

 cells of the first species are mixed with cells of one of the 

 other species ; it is then impossible, judging from the form 

 alone, to distinguish the larger and smaller oval and round 

 cells of the Pastoriunus species from many of the cells of Sacch. 

 cerevisice. The two species Sacch. ellipsoideus I. and //. consist 

 mainly of oval and round cells ; sausage-shaped cells, however, 

 also occur ; and thus it is in this case impossible to determine 

 the species by the form of the cells when these are mixed with 

 Sacch. cerevisice or Sacch. Pastorianus. 



