288 XXI. BACTERIA ?EAST. 



and may carry on the manufacture of what we may call " bye- 

 products". More than once we have referred to the development 

 of characteristic colours, smells, etc., as products of the chemical 

 activity of the bacteria. Actively concerned, then, as they are in 

 the breaking down of organic chemical bodies, many of them carry 

 on their nutrition by manifestly inducing changes which it has 

 become convenient to speak of as " fermentative ". 



Life History of Yeast. We will now make the acquaintance of 

 another fungoid organism, likewise of very simple character, the 

 whole life cycle of which we can study by means which our labours 

 with the bacteria have made us conversant with, and in which 

 fermentative action, of another kind however, is very strongly 

 marked. These are the very simply-constructed fungal organisms 

 hitherto collected together under the name of Saccharomycetes. 

 We provide ourselves with some Yeast, the " barm " used in 

 brewing beer, and examine a trace of it, diffused in water, under a 

 high power. We find the field of view filled with small cells, 

 individuals of the so-called yeast fungus, Sacckaromyces cerevisece. 

 The cells appear globular or elliptic ; they have a delicate membrane, 

 and in the interior can be recognised a large or 

 several small vacuoles, and some highly refrac- 

 tive granules. A nucleus can only be distinguished 

 3 by the aid of hardening and staining, and then 



FIG. 106. Sacch- w jth difficulty. Numerous cells are seen in 



aromyces cerevisice. ~ . 



i, not budding ; 2 course of multiplication. This takes place in a 

 ?x ( 540) bUdding Ce " S q uite characteristic and peculiar fashion, by the 

 cells forming one, rarely more, small knob-like 

 swellings, which gradually attain the size and form of the mother- 

 cell, and then are separated from it by a partition wall. In very 

 energetic development we find the daughter-cells united into 

 small, occasionally branched, chains ; in slower development the 

 cells separate before any new one commences to form. This is 

 multiplication by budding, peculiar to the Saccharomycetes. In 

 sugar-containing fluids, yeast brings about alcoholic fermentation. 

 The individuality of the Saccharomycetes has been much ques- 

 tioned, and they have been declared to be conidia (that is spores, 

 of a kind), of different fungi, which have the power, in a suitable 

 nutrient fluid, of multiplying by budding in indefinite sequence. 



The pure culture of yeast has now a very high technical 

 importance. The great breweries work only with pure yeast, 

 and the same is needed now a days for the fermentation of the 



