152 MICRO-ORGANISMS AND FERMENTATION. 



were formerly described under the general names Sacch. 

 Pastorianus, Sacch, ellipsoideus, &c.), but also of well- 

 characterised top- and bottom-yeasts, which are employed in 

 practice. It is a point of great practical interest that 

 species cultivated in beer-wort, the cultivation of which has 

 been uninterruptedly continued for several years, have shown 

 no, or at most but slight, changes. At the same time that 

 Hansen arrived at these results, he also discovered that it 

 was possible, by suitable treatment, to produce variations in 

 different directions ; also the individual peculiarities of the 

 cells in an absolutely pure culture can here assert them- 

 selves. Some of these changes are only temporary, and 

 disappear under suitable treatment when the species re- 

 assumes its original character. Others become more deeply 

 rooted, and it is then only under especially favourable con- 

 ditions that the culture can be deprived of its newly- 

 acquired properties. In certain cases it was not possible, 

 even after years of methodical treatment, to re-convert a 

 culture to its original state. 



1. As is known, the data regarding the time required 

 for the appearance of the first indications of spore-forma- 

 tion in the previously-described six species, are subject to 

 the condition that the growth has been previously cultivated 

 in wort for 24 hours at a temperature of 25 C. At the 

 same time that Hansen published (1883) the temperature 

 curves for these six species, he also found that cultures 

 which had been grown in wort at the above temperature, 

 but for two days instead of one, developed spores more 

 slowly and more sparingly than usual. If, however, such 

 cultures are subsequently treated in the manner first 

 described, the culture again assumes its normal condition. 

 We have here, therefore, an example of a very feebly-rooted 

 variation. 



2. In a gelatine culture of " Carlsberg bottom-yeast 

 No. 1 " both oval and elongated sausage-shaped cells are 

 often found, so that according to Reess the presence of two 



