222 THE ORGANISMS IN YEAST, 



absence of ferment-life, and when it is desired to start a culture, 

 the material is introduced at the side-neck, and the flask or flasks 

 being retained at a suitable temperature, the development rapidly 

 takes place. The basis on which the culture proceeds, and the 

 means to be adopted in order to ensure reliable results, having 

 been dealt with, it remains to consider what it is which we are 

 about to cultivate in these carefully-prepared flasks. 



Yeast, in its ordinary form and when fresh, consists of a light, 

 yellowish, more or less pasty mass, composed mainly of minute 

 unicellular organisms, mixed with a certain amount of the ferment- 

 ing liquid from which it has been removed, and more or less car- 

 bonic-acid gas. It is with the organisms that we have now to do. 

 These consist, firstly and chiefly, of the "Yeast-Plant" of Schwann, 

 the cells of the Torula cerevisice. or, as it has more recently been 

 called, the Saccharomyces cerevisice^ which are shown in Plate 25 

 at Nos. I and 2, as seen under a quarter-inch objective, and in 

 Nos. 3, 4, 5, and 6 under a one-tenth immersion objective, with 

 No. 3 oc, together usually with certain nearly allied species of 

 Saccharomyces, and of a few cells of the Badermm lactis (shown in 

 the Plate at No. 8, as seen under the one-tenth inch objective). 

 But usually one finds also more or less of those varieties of fer- 

 ment-life which Pasteur has designated as the " ferments of 

 disease " {^ferfiients de maladie). Those which are most commonly 

 met with, in addition to the lactic cell — which, unless present in 

 large quantity, is not generally considered as injurious — are the 

 thin acetic rod, or filament, which produces acetic acid (No. 7), 

 and more rarely the butyric ferment, the Bacillus siihtilis (No. 9), 

 the minute SaLxharomyces apicidatus^ (No. 12), and other forms of 

 less importance. 



Before proceeding to describe these, it will be as well to glance 

 for a moment at the various views of the nature and classification 

 of these organisms which have been advanced by different observ- 

 ers. Von Naegeli, in a valuable work, pubHshed in 1877, con- 

 siders them all as belonging to the Fungi, and as nearly related to 

 the moulds or mucedines, although, as he points out, this connec- 

 tion is only traceable so far as those organisms of which the Torula 

 cerevisicB may be taken as the type. He divides them into two 

 groups only, making the "moulds''" into a third, and describes 



