290 MICRO-ORGANISMS AND FERMENTATION. 



Morphology and Anatomy of Yeast Cells. 



Yeast Deposits. Hansen's investigations in 1881-1883, 

 which took the form of a direct study of the growth of a single 

 cell under the microscope, and of growths derived from a 

 single cell, made it possible for the first time to give exact 

 descriptions of the different species of yeast. He proved 

 that the shape, relative size, and appearance of the cell are 

 not sufficient in themselves to characterise a given species, 

 for the same species may exist in different forms under differing 

 external influences. At the same time he established the fact 

 that the shape may provide valuable indications, as the 

 various species may react in a different way and with a 

 different shape when the same influence is brought to bear. 



As an example of the results which may be obtained by 

 a comparison of young deposits of yeast, the six varieties 

 isolated by Hansen may be quoted (8. cerevisice I., S. Pastori- 

 anus /., //., ///., 8. ellipsoideus /., //.). 



The growths are developed in the following manner : The 

 cells, after short cultivation in wort, are introduced into fresh 

 wort, and brought to vigorous development at 25 to 27 C. 

 in twenty-four hours. If then 8. cerevisice /. is compared with 

 the three 8. Pastorianus species, the general appearance -is 

 strikingly different. 8. cerevisice I. consists predominantly of 

 large round or oval cells, and 8. Pastorianus chiefly of elongated 

 sausage-shaped cells, but it is a very different matter if the 

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

 species. It then proves to be impossible, by simply noting 

 the form, to distinguish between the larger and smaller oval 

 and roundish cells of Pastorianus and many of the cerevisice 

 cells. The two species, 8. ellipsoideus I. and //., are pre- 

 dominantly oval and round. Sausage-shaped cells occasionally 

 occur, and here again it is impossible, simply by studying the 

 form, to determine the species when 8. cerevisice or 8. Pastori- 

 anus are mixed with them. 



By direct measurement of the sedimentary forms it is also 

 impossible to discriminate them. 



On examining pictures of these six pure cultures, it will 

 be seen that we are dealing with three different divisions of 



