THE YEASTS CHAPMAN 307 



to which I have referred above — into five subgroups, each of which 

 again is capable of subdivision into a considerable number of well- 

 defined species. 



With regard to film formation, this again is a character of 

 considerable importance in connection with the differentiation of 

 species. As I have indicated, certain of the yeasts — especially 

 those included in the fourth of the nuiin groups above referred to — 

 grow normally on the surface of nutrient liquids, forming dry 

 greasy films, and producing, as a rule, little or no alcohol. It is 

 not, however, of such films as these that I am now speaking. 

 Although the true Saccharomycetes do not readily exhibit the 

 phenomenon of film formation, yet under proper conditions this 

 is capable of being brought about. In order to obtain films, the 

 culture, in wort for example, must be allowed to remain at rest 

 for a considerable time with an abundance of air, and at Avarm 

 room temperatures. Under these circumstances small specks of 

 yeast will generally appear on the surface of the liquid some con- 

 siderable time after the actual fermentation has come to an end. 

 These gradually coalesce, forming isolated patches of different 

 forms and sizes, and, finally, these patches unite to form a con- 

 tinuous and generally glutinous film which may extend to the walls 

 of the vessel. A study of the time and temperature factors involved 

 in the formation of these films affords, as I have indicated, one 

 method of distinguishing between certain yeast species. So far as 

 the purely morphological differences are concerned, it was observed 

 at a comparatively early period that the yeasts exhibited consid- 

 erable differences in shape, some being spherical or ovoid, whilst 

 others were decidedly ellipsoidal, and others, again, elongated and 

 sausage-shaped. On the other hand, in one and the same species 

 very great differences in shape and size were frequently noticed, 

 depending upon the general environment and growth conditions of 

 the organism. No great advance, therefore, could be made in dif- 

 ferentiating between the various species until Hansen, in 1879, 

 devised and explained the technique for obtaining any quantity of 

 yeast by starting with a single cell. In this way absolutely pure 

 cultures could be obtained, and the uncertainty which had previously 

 existed was removed. 



Of the very large number of yeast species known, it may be said 

 at once that only a comparatively few are of industrial importance, 

 and it is customary to divide the various yeast species for technical 

 purposes into the " cultivated " and the '' wild " yeasts. The former 

 include brewers' and distillers' yeast in all its varieties — that is to 

 say, yeast which has from the earliest times been used for the pro- 



