GELATIN AND POTATO MEDIA. 189 



hanging-drop examinations and stained preparations ac- 

 cording to the directions already given. 



The Stab Culture in Gelatin. 



The object in making plate cultures is to obtain colonies 

 which, since they are derived from a single cell, are pure 

 cultures of that organism. To perpetuate and keep up the 

 pure culture thus obtained, it is necessary to resort to trans- 

 plantation. For this purpose, the colony to be transplanted 

 is touched with a sterilized and cooled, straight platinum 

 wire. A portion of the colony will adhere to the end of the 

 wire, and can be transferred to a tube of sterilized gelatin. 

 The wire is usually pushed down the center of the solid 

 gelatin, in which case we have what is known as a stab 

 or stich culture. The operation of touching the colony 

 is one that requires the greatest care to prevent con- 

 tamination with foreign colonies, or with other material, 

 since this would vitiate the pure culture. For this reason 

 it is always carried out under a microscope, and so far 

 as patience is concerned, it certainly is not inaptly called 

 " fishing." 



The gelatin plate is placed on the stage of the micro- 

 scope, and, with the No. 3 objective, a suitable colony for 

 transplantation is selected. It is desirable to have but one 

 colony in the field of the microscope. A straight platinum 

 wire, previously sterilized and cooled, is held in the right 

 hand in the pen position. The hand is supported by rest- 

 ing the little finger on the right corner of the stage. The 

 platinum wire is then inserted, about midway, between the 

 front lens of the objective and the surface of the gelatin. 

 It is held steadily in this position, and on looking into 

 the microscope an indistinct shadow is seen. The wire 

 is slowly drawn back till the end of the shadow, or in- 

 distinct wire is directly over the colony. Should the wire 



