30 



the ice-machine, plates, benches, plate boxes, etc., is done 

 away with. 



Esmarck Roll- Tube Cultures. In this method the 

 advantages of the plate method are secured without the 

 use of any extra apparatus, as plates or dishes. The 

 inoculated gelatin instead of being poured out onto steril- 

 ized 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. Desiccation of the gelatin can be 

 readily prevented in the roll- tube, whereas it is much 

 more difficult in plate or dish cultures. 



Inoculate three gelatin tubes with the material to be 

 plated, in the usual manner. Then cut off the cotton 

 plug on each tube and cover the end with a rubber cap. 

 Place the tube in a horizontal position, or nearly so, in a 

 dish of cold, or ice-water and rotate it carefully until the 

 gelatin solidifies in an even thin layer over the inside wall 

 of the tube. Avoid contact of the gelatin with the cotton 

 plug. Now set aside in a cool place to develop and then 

 examine the colonies under a microscope and make trans- 

 plantations. The operation of fishing in this case will, of 

 course, require special care. 



LABORATORY WORK. Make Esmarch roll-tubes of 

 saliva (1 loopful); and of the Violet bacillus of water. 



Make Petri dishes of the Red bacillus of water. 



MODIFIED POTATO CULTURES. 



The method of making potato cultures of bacteria is 

 open to much the same objections as the ordinary gelatin 

 plate method. Two modifications, analogous to the two 

 modified gelatin plate cultures, are commonly employed 

 and are excellently adapted for their purpose. 

 . . Esmarch Potato Cultures. The Esmarch dishes, 

 which are about 6 cm. in diameter and 2cm. high, are ster- 

 ilized in the dry heat oven in the usual manner, and then 

 allowed to cool. A small, sound potato is selected and 



