240 APPLIED BACTERIOLOGY 



following is a brief account of the methods employed by 

 Hansen to determine the characteristics of a species : 



(a) The Microscopic Appearance of a Yeast. — The growths, 

 after growing in sterilised wort for twenty-four hours, are 

 examined under the microscope. The general characteristics 

 are then noted ; for instance, whether the cells are round 

 or oval, as is the case with the S. cerevisice, or elongated 

 sausage-shaped cells, as in the case of the Sacch. pas- 

 torianus varieties. It is a very different matter, however, 

 when these two species are mixed, or other varieties are 

 present. In this case but little is to be learnt from a 

 direct microscopic examination. 



(6) The Formation of Ascospores. — By the determination 

 of the temperature and time necessary for the various 

 species to form ascospores, Hansen made the first step in 

 devising an analytical method for the examination of yeasts. 

 After making a large number of experiments, Hansen was 

 able to determine the following conditions which regulate 

 the formation of spores in the saccharomycetes : 



(1) The cells must be placed on a moist surface, and 

 have plenty of air. 



(2) Only young and vigorous cells can exercise this 

 function. 



(3) The most favourable temperature for most of the 

 species is about 25° C. 



(4) A few saccharomycetes form spores when present in 

 fermenting nutrient liquids. 



A small portion of a young and vigorous growth is trans- 

 ferred to a moist gypsum block, prepared as follows : To 

 well-baked plaster of Paris add distilled water until the 

 plaster is nearly liquid ; pour this on to a sterilised glass 

 plate on which rests a small mould of metal or paper. 

 The blocks are dried and sterilised. They are then laid 

 in a shallow tray coiitainiag a little sterile water, the 

 whole arrangement being kept well covered by a bell-jar. 



