58 



GENERAL BACTERIOLOGY 



fied in a thin, even layer on the inner surface of the test-tubes. This 

 is best accomplished by means of a piece of ice placed in a dish on 

 a piece of cloth, by which it can be kept in the desired position 



(Fig. 18). A hori- 

 zontal groove is melt- 

 ed in the jce by means 

 of a test-tube filled 

 with hot water. In 

 this groove the test- 

 tubes, inoculated as 

 in case of plate cul- 

 tures, are rapidly 

 whirled until the me- 

 dium is thorovighly 

 set. Both agar and 



Pig. 18. Method of making Roll-cultures. (Abbott). gelatin can be used, 



although gelatin cannot be used successfully with those species which 

 liquefy this medium. In the case of agar the tubes should be placed 

 in a horizontal position a few hours (over night) until the medium 

 has become attached to the tube ; afterwards they can be stored in 

 the usual receptacles for tube cultures. 



Repeeences. a. ] 37 ; H. 69 ; M. & R. 56 ; McF. 206. 



Special Directions, a. Melt. a tube of gelatin and without inoc- 

 ulating it practice making a roll-culture as described above. Avoid 

 tipping the tube enough to get medium on cotton plug. Remelt and 

 roll again and again until the knack is acquired. 



h. Make two roll-cultures in gelatin of B. coli (13), using a water- 

 blank instead of gelatin tube No. 1. 



c. Make two agar cultures of B. suhtilis in same way. 



d. Incubate h. in cool chamber, and c. at 28° C. 



EXERCISE 33. STUDY OP PLATE CULTURES. 



Macroscopic. As the colonies appear, note : a. form, h. size, 

 c. surface elevation, d. consistency, e. color. Both the surface and 

 deep colonies should be described, as they are frequently very differ- 

 ent. Drawings should always be made wherever they will be of 

 value ; study should be continued as long as changes are noticed. 

 (See Chapter III.) 



MiCEOSCOPic. The colonies appearing on the plates are to be 

 studied under a low power of the microscope. Use a f in. (16 mm.) 



