GELATIN AND POTATO MEDIA. 181 



When pouring the gelatin, the cover of the Petri dish 

 should not be raised any more than is absolutely necessary. 

 The cover is then replaced, and the dish is gently tilted, 

 from side to side, so as to cause the gelatin to spread over 

 the entire bottom. The dishes are then placed on a flat 

 surface, and, when the gelatin has solidified, they are set 

 aside in order to allow the colonies to develop. 



In order to make a preliminary examination of the 

 colonies, the dish is inverted on the stage of the micro- 

 scope. The colonies can thus be studied almost as well as 

 if the top was removed. In order to make transplantations 

 from the colonies, it will be necessary, of course, to remove 

 the top, and to place the bottom upright on the stage of 

 the microscope. 



In this method, each dish constitutes a plate and a 

 moist chamber. The progress in development can be ob- 

 served without exposing the gelatin to the air, and with- 

 out any risk to the other plates. Consequently, the chance 

 of contamination is reduced to a minimum. There is little, 

 or no danger of the gelatin dripping on the floor, table, or 

 stage of the microscope. Moreover, the use of the ice ap- 

 paratus, of plates, benches, boxes, etc. , is done away with. 

 This method of obtaining colonies is to be used, whenever 

 possible, in preference to the original Koch method. 



Esmarch roll-tube method. — In this the advantages of the 

 plate method are secured without the use of any extra ap- 

 paratus, such as plates or dishes. The inoculated gelatin, 

 instead of being poured out on sterilized plates, or into 

 dishes, is solidified in a thin film on the inside wall of the 

 test-tube. Another advantage of this method is, that it is 

 well adapted for those organisms which grow very slowly, 

 and require a week or two to form distinct colonies. Desic- 

 cation of the gelatin can be readily prevented in the roll- 

 tube, whereas this is more difficult to do in plate, or in 

 dish cultures. 



