EXAMINATION OF YEASTS 315 



necessary to take a single cell, and from this to grow a 

 series of cells in sterilised beer-wort, to break this up into 

 different portions of seed-material, from which other crops 

 are produced, and so on, until a sufficient quantity of pure 

 yeast is produced to bring about the necessary fermentation 

 in a large' bulk of wort. 



The characteristic appearance which at one time was 

 thought to belong to the yeast-plant has been shown by 

 Hansen to have no existence, except in a very limited sense. 

 Throughout the entire series of Hansen's researches the 

 leading idea obtains— namely, that the shape, relative 

 sizes, and the appearances of the cells, taken by them- 

 selves, are not sufficient to characterise a species. The 

 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 Sacch. 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. 



(b) 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. 



