78 A MANUAL OF BACTERIOLOGY 



When suitable, sterile nutrient gelatin is usually 

 employed for the preparation of plate cultivations, as 

 it is more easily manipulated than agar. Three tubes 

 of sterile nutrient gelatin are melted at a low tempera- 

 ture in a beaker of water (gelatin melts at 24 C. ; the 

 temperature should not exceed about 45 C.). The tubes 

 may be termed respectively 1, 2, and 3. Tube No. 1 is 

 inoculated, by means of a platinum needle, with a trace 

 of the growth from which pure cultivations are desired. The 



trace of growth is thoroughly 

 mixed up and distributed 

 throughout the melted 

 gelatin. If this mixture be 

 " plated," so many organisms 

 may be present in the film 

 that the colonies which de- 



lP wiH "<* ^ separate, 

 but will form a confluent 



growth. To obviate this difficulty a second and a third 

 dilution are prepared. The second dilution is made 

 by inoculating the tube of melted gelatin No. 2 with 

 one platinum loopful from tube No. 1, and thoroughly 

 mixing up ; and to be quite sure that the resulting 

 colonies will be isolated from one another, a third dilu- 

 tion is prepared in the same manner by inoculating the 

 tube of melted gelatin No. 3 with two to four platinum 

 loopfuls from tube No. 2. The organisms having been 

 distributed throughout the gelatin by rolling and gentle 

 shaking, the wool plug is in each case withdrawn from 

 the mouth of the tube, the mouth of the tube is sterilised 

 in the Bunsen burner to prevent contamination, then 

 cooled for a few seconds, and finally the melted gelatin is 

 poured on to a level sterile glass surface. Formerly plates 

 of glass were used (hence the name) ; but now shallow 

 glass dishes with lids, about three or four inches in 



