THE ORGANISMS IN YEAST. 225 



breweries of the West of England. In some of the specimens, 

 and notably in the one from which the drawing was taken, it 

 endured for at least a week. The nutrition of the cell appears to 

 be effected by a process of osmotic diffusion through the cell-wall, 

 a continual transmission of nutrient material going on from the 

 surrounding liquid towards the interior, where it is assimilated as 

 food, and a reverse or outward action, by which we may presume 

 some excretory products are removed, simultaneously proceeding. 

 Hence we may assume that only diffusable substances are capable 

 of sustaining the life of the Yeast-cell, but how far it possesses 

 the power of rendering materials which may be presented to it fit 

 for its assimilation and sustenance is another thing, and one 

 which, though deeply interesting, I must pass by for the present. 



The presence of oxygen gas, or of some carbo-hydrate or other 

 compound, by the decomposition of which oxygen can be obtained, 

 and of some protein material, in this nutrient medium, from which 

 the cells can derive nitrogen in some form, are essential to its life 

 and development. The exact condition in which this nitrogenous 

 nutriment is absorbed is a matter of considerable technical 

 importance, and is at present occupying my attention. The multi- 

 plication of the cell takes place by a process of geminatio7i^ or 

 budding, and though it is not unlikely that considerable additional 

 knowledge on this head will be gained in the future, yet it would 

 seem that if any other mode of propagation exists, it very rarely 

 comes into operation. The parent cell, when placed in a medium 

 affording a supply of readily-assimilated nutriment, rapidly assumes 

 a fuller and more transparent appearance, and after a time extrudes 

 a bud of protoplasm, which, itself absorbing sustenance like the 

 parent, soon equals it in size, and puts forth a second generation — 

 if that word may be allowed for such a process. Thus, if food be 

 abundant, and the other conditions not unfavourable, a rapid 

 increase of cells takes place, and as they rarely part company 

 immediately, they often form chains of considerable length, half-a- 

 dozen or more remaining together. This process is shown in the 

 drawings in the upper part of the plate, Nos. 4, 5, and 6 of which 

 show the changes taking place in a fermenting wort at intervals of 

 forty-eight hours. The result of the vital action of the cell is the 

 well-known formation of alcohol and carbonic acid, and certain 



