LABOBATOEY WOBK WITH ANTHEAX ANIMALS. 285 



Agar plate culture. — The g'elatin plates, prepared as 

 above, do not always yield the most characteristic col- 

 onies. Moreover, owing to the liquefaction of the gela- 

 tin, it is difficult to obtain satisfactory cover-glass im- 

 pressions. The agar plates, on the other hand, give 

 excellent colonies from which impression preparations 

 can be easily made. The method of making agar plates 

 is as follows: 



Three agar tubes, each of which contains about 8 c.c. 

 of agar, are selected and placed upright in boiling water. 

 When the agar has become perfectly fluid, the burner is 

 then removed from under the water-bath, and the water 

 with the immersed tubes is allowed to cool slowly until a 

 temperature of 50° is reached. Tube 1 is then inoculated 

 with a piece of the liver, or other organ, in the manner 

 described above when making gelatin plates. The usual 

 dilutions to tubes 2 and 3, are then made as rapidly as pos- 

 sible. After withdrawal of the cotton plug, the neck of 

 each tube is flamed and the contents poured into sterile, 

 Petri dishes. The plates are then set aside in the incuba- 

 tor at 37° for 12-18 hours. 



The nutrient agar solidifies at about 40°. Conse- 

 quently, rapid work is necessary in order to inoculate the 

 tubes and pour the contents before solidification takes 

 place. If this does occur, the experiment must be repeated 

 with new agar tubes. Ice-water must not be used to con- 

 geal the agar plates. They will solidify without employ- 

 ing any cooling apparatus. 



When developed, the colonies should be compared with 

 those on gelatin; they should be sketched and used for 

 making impression preparations. 



Hanging-drop examination. — Take a clean f inch cover- 

 glass and sterilize by passing it several times through the 

 flame. Transfer a small drop of sterile bouillon to the 

 cover-glass and then add to it, with a sterilized wire, a min- 



