186 BACTERIOLOGY. 



The inoculation of the sterilized potato tubes is easily 

 done. If it is desired to obtain dilution cultures, that is, 

 to say, colonies, this can best be accomplished by making 

 several parallel streaks on the surface of the potato with 

 the end of a straight platinum wire. The same wire, un- 

 sterilized, is then streaked repeatedly over the surface of 

 a second potato, and then over that of a third. The latter 

 will undoubtedly have but a few cells planted upon its sur- 

 face, and, when these multiply, they will yield isolated 

 colonies. 



When transplanting- a pure culture, such as a portion 

 of a colony, a single streak should be made along the mid- 

 dle of the inclined potato. The characteristics of the re- 

 sultant streak should be carefully noted. 



Laboratory work. The student will prepare 20 or 30 potato tubes 

 according- to the directions given above. Streak cultures will be 

 made of each of the various organisms studied. 



Examination of Colonies. 



If the temperature is about 22 the gelatin plates will 

 probably show signs of growth in 24 hours. As a rule, 

 however, 2 or 3 days must be allowed for the development 

 of colonies. These become visible to the eye, first as mere 

 white points; eventually, they enlarge and attain the size 

 of a pin-head, or become even larger. A careful study of 

 the colonies on the several plates should first be made with 

 the unaided eye. This macroscopic examination is very im- 

 portant. The eye should be taught to detect the first sign 

 of a liquefaction, the early appearance of pigment, the 

 various differences in the form, size, color and structure of 

 the colonies. In this way important information may be 

 gained which will enable the subsequent work to be done 

 to advantage. The various characteristics can often be 

 recognized at quite an early period. 



