GELATIN AND POTATO MEDIA. 169 



therefore, as the next step, a pin-head portion is removed by 

 means of a sterile knife from the surface of this first potato 

 (No. 1) it will contain a very small fraction of one per cent, 

 of the total number of bacteria spread over the surface of 

 that potato. This relatively small number of bacteria is 

 now spread, or distributed, as thoroughly as possible, over 

 the surface of a second potato. In doing- this the same 

 precautions are taken as when potato No. 1 was inoculated. 

 When the bacteria that are thus planted on the second 

 potato (No. 2) develop they may still be so numerous as to 

 give rise to a mass culture. Should only a few bacteria be 

 present they will be scattered over the surface, separated 

 by an appreciable distance, and, when they develop, they 

 will give rise to isolated growths or colonies. Inasmuch 

 as the result with the second potato is uncertain it is cus- 

 tomary to take, by means of a sterile knife, a small portion 

 of material from the surface of potato No. 2, and, to spread 

 this over that of a third potato (No. 3). The latter will 

 probably have planted on its surface some 10 or 20 bacteria. 

 These may be i to | an inch apart. The bacterial cell, 

 wherever deposited, grows and multiplies, and eventually 

 forms a visible, pin-head growth. Ttyis isolated growth is 

 known as a colony, and, inasmuch as it is derived from a 

 single cell, it is a pure culture of that organism. 



By resorting to dilution cultures, in the manner indi- 

 cated, it is possible to isolate the various bacteria that may 

 be present in the original material. To illustrate the dilu- 

 tion that takes place it may be assumed that 10,000,000 cells 

 were planted on potato No. 1. The small portion taken 

 from this may have planted 10,000 bacteria on potato No. 2. 

 Similarly, the small amount taken from the second and 

 planted on the surface of the third potato may not contain 

 more than 10 cells. In 36 to 48 hours, if the temperature is 

 favorable, these cells will have multiplied to such an extent 

 as to give rise to visible growths or colonies. 



The simple method of dilution, as just described, is the 



