TEST-TUBE, STAB- AND SMEAR-CULTURES 155 



from the inoculation of this bit of colony into a fresh sterile 

 medium will be pure. 



In the meantime have in the other hand a test-tube of 

 sterile medium: gelatin, agar-agar, or potato. This tube is 

 held across the palm of the hand in an almost horizontal 

 position with its mouth pointing out between the thumb 

 and index-finger and its contents toward the body of the 

 worker. With the disengaged fingers of the other hand 

 holding the needle the cotton plug is removed from the 

 tube by a twisting motion and placed between the index 

 and second fingers of the hand holding the tube, in such a 

 way that the portion of the plug which fits into the mouth 

 of the test-tube looks toward the dorsal surface of the hand 

 and does not touch any portion of the hand; this is accom- 

 plished by placing only the overhanging portion of the plug 

 between the fingers. The needle containing the bit of 

 colony is now to be thrust into the medium in the tube if 

 a stab-culture is desired, or rubbed gently over its surface 

 if a smear or stroke-culture is to be made. The needle is 

 then withdrawn, the cotton plug replaced, and the needle 

 sterilized before it is laid down. Neither the needle nor its 

 handle should touch the inner sides of the test-tube if it 

 can be avoided. The tube is then labelled and set aside for 

 observation. The growth which appears in the tube after 

 twenty-four to thirty-six hours should be a pure culture of 

 the organisms of which the colony was composed. 



Cultures of this form are not only useful as a means of 

 preserving the difFerent organisms with which we may be 

 working, but serve also to bring out certain characteristics 

 of different organisms when grown in this way. 



If gelatin be employed and the organism which has been 

 introduced into it possesses the power of bringing about 



