106 BACTERIA. 



most vigorous at all temperatures, at 13 C. it develops so 

 quickly and vigorously that it out-distances all the others, 

 giving at 23 C. a film covering the whole of the surface in from 

 six to twelve days ; whilst the others at the same tempera- 

 ture only gave a much more delicate film in from three to 

 five weeks. The next to this as regards rapidity of the forma- 

 tion of the film is Sacch. Pastorianus III., which at the 

 temperature of a warm room forms a film much more rapidly 

 than any except Sacch. ellipsoideus II. 



It may be noted, in connection with the limits of tempera- 

 ture at which this film formation takes place in the three 

 Pastorian varieties, that they cease to develop and form 

 spores at a temperature of 36 to 38 C., if this be con- 

 tinued for ten or eleven days ; whilst the other three species 

 all continue to grow for a much longer period. 



In addition to the forms studied by Hansen others may 

 be mentioned. Below is appended a short classification of 

 the more important forms of yeasts, with some of their more 

 characteristic features. 



The Saccharomycetes or yeast fungi are a family of 

 ascomycetes, divided into two very unequal groups the 

 Saccharomyces of Reess and the Monaspores of Metschnikoff ; 

 the mode of spore formation is somewhat similar in the 

 two, the only difference being that in the saccharomyces the 

 spores are rounded and are sometimes multiple in the same 

 sporangium, whilst in monaspora there is in each cell a 

 single needle-shaped spore developed. In the first genus are 

 included the following forms : 



i. Saccharomyces cerevi'stce, may be looked upon as a 

 typical English high yeast, some of the characteristics 

 of which have already been described. It grows as rounded 

 or slightly ellipsoidal cells, which give off small cells by 

 budding ; in the earlier stages of film formation there are 

 formed delicate mycelial-like threads, but as the film becomes 

 older longer and more regular threads are formed. In the 

 yeast-cells nuclei are frequently to be made out, especially on 

 being stained with osmic acid or haematoxyline. These nuclei 

 are very distinctly seen in old cultivations. 



The development of ascospores takes place most rapidly (after twenty hours) 

 at 3oC., most slowly (after ten days) at from 11 to 12 C., and stops alto- 

 gether below this. The spores may be very distinctly seen, as they are highly 

 refractile, and their walls are well defined. They are usually from 2.5 to 6/t 



