CHARACTERISTICS OF CULTIVATIONS OF FISSION FUNGI. 165 



containing much water, is especially characteristic. 

 Solid substrata have of late been employed by preference Advantages of 

 for the cultivation of the bacteria, and they are useful chiefly foVthe reeog? 

 because they permit a completely independent develop- 

 rnent of each individual bacteric colony, while in fluids 

 the individual species do not grow at a definite spot, but 

 become mixed with any other species which may happen 

 to be present. It is therefore only on solid substrata 

 that the characteristic appearances of the colonies formed 

 by any given species of bacteria can be observed, while 

 it is just the external appearances of the isolated pure 

 colonies which are of the greatest importance, because 

 they differ in almost every species of bacteria, and be- 

 cause we can thus obtain much more important charac- 

 ters and better distinctive marks than we get with the 

 help of the microscopical differences in form. For this 

 reason the appearance of the isolated colonies is of special 

 service in the diagnosis of the species of bacteria, a thing 

 which is otherwise often very difficult. 



As suitable solid substrata we employ either potatoes, 

 bread-paste, &c., on which the bacteric colonies appear, 

 sometimes as slimy white, yellow, or reddish drops, 

 sometimes as drier whitish films, sometimes as diffuse, 

 gelatinous layers of various colours, and sometimes as 

 delicate, scarcely noticeable skins ; or we use the so- 

 called nutrient jelly (see culture methods), which is 

 prepared by adding so much gelatine to a good nutrient 

 solution, that the mixture forms a transparent mass, 

 which is solid below 25 C. This nutrient jelly is 

 employed for plate cultivations, in which a very small 

 portion of the bacteric mixture to be investigated is 

 added to a quantity of the nutrient jelly, which is 

 liquefied by placing it at 30 C. : after the admix- 

 ture is complete it is then poured out on a horizontal 

 glass plate, so that the latter is covered by a layer a few 

 millimetres deep. The jelly solidifies quickly ; each Character of 

 individual bacterium in the mixture is thus fixed at a 

 definite spot, and grows there to form a colony. If the plates. 

 experiment is successful, i.e., if too many bacteria 

 have not been mixed with the quantity of jelly, and if 



