26 



colonies should now be made. This is done by making 

 hanging drop examinations and stained preparation accord- 

 ing to the directions already given. 



The object in making plate cultures is to obtain colo- 

 nies 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 transplantation. 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 centre of the tube, in which case we have what 

 is known as a stich or stab culture. The operation of 

 touching the colony is one that requires the greatest care 

 to prevent contamination with foreign colonies, or other 

 material, thus vitiating 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 indistinct 

 wire is directly over the colony. Should the wire in doing 

 this touch the objective or the gelatin it must be sterilized 

 at once and the operation repeated. When the end of the 

 wire has been brought over the colony, gradually lower 

 the point till it touches the colony or cuts it into two. 



