TIIK 1'RErAKATION OF FARM PRODUCTS. 285 



investigation has appeared to increase the number of species 

 very greatly. In the last few years many different organisms 

 have been added, until to-day the list of organisms, regarded as 

 distinct species, and having properly the designation of acetic 

 bacteria, is very long. 



The last few years have seen a large amount of study given 

 to these various acetic bacteria. Not only have the species 

 been multiplied and the characters of the different forms care- 

 fully studied, but numerous physiological experiments have been 

 made upon their power of producing acetic acid from different 

 materials and under different conditions. The great multipli- 

 cation of species has led to a belief that the various forms are 

 not to be regarded as distinct species, but simply as variations 

 of a few types under different conditions. To-day there is a 

 tendency to reduce them all to a few central types. Exactly 

 what will be the result of the study of species can hardly be 

 stated at the present time. 



All of these acetic bacteria have certain very characteristic 

 points which they share in common, a fact indicating that they 

 form a distinct class of bacteria by themselves. They all 

 exist in three different forms, shown in Figs. 34 to 36. 

 They may form chains of short rods, looking like ordinary 

 bacilli (Fig. 34). They sometimes grow out into long, slender 

 threads, sometimes very long without any traces of divisions. 

 These threads may subsequently break up into short elements. 

 Finally they all have a peculiar habit of forming long threads 

 with rounded, swollen centers (Fig. 36). When these threads 

 break up into short forms, only the thick, swollen centers re- 

 main undivided. What significance these swollen centers have 

 in the life of the organism is not understood. This character 

 is such a peculiar one, and so unique, that it plainly places 

 these acetic bacteria in a class which must remain isolated 

 from other bacteria. It is a class of organisms with quite a 

 large number of different varieties, distinguished not only by 



