198 



Cultures, and Their Study 



forceps (Fig. 43) adapted to holding them having been 

 devised by Rosenberger.* 



The dilution of the material under examination is made 

 with gelatin or agar-agar tubes in the manner above de- 

 scribed, the plug is removed, the mouth of the tube cau- 

 tiously held for a moment in the flame, and the contents 

 poured into one of the sterile dishes, whose lid is just suffi- 

 ciently elevated to permit the mouth of the tube to enter. 

 The gelatin is spread over the bottom of the dish in an 

 even layer, allowed to solidify, labeled, and stood away for 

 the colonies to develop. 



Among the other advantages of the Petri dish is the 

 convenience with which colonies can be studied with a low 

 power lens. To do this with the Koch plates meant to re- 

 move them from the sterile chamber to the stage of a micro- 

 scope and so expose them to the air, and to contamination, 

 but to examine colonies in the Petri dish, one simply inverts 

 it without opening and examines through the thin glass of 

 the bottom dish without any exposure to contaminating 

 organisms. 



Es march's Tubes. This method, devised by Esmarch, 

 converts the wall of the test-tube into the plate and dispenses 



Fig. 44. Esmarch tube on block of ice (redrawn after Abbott). 



with all other apparatus. The tubes, which are inoculated 

 and in which the dilutions are made, should contain less than 

 half the usual amount of gelatin or agar-agar. After inocu- 

 lation the cotton plugs are pushed into the tubes until even 



*"Phila. Med. Jour.," Oct. 20, 1900, vol. vi, No. 16, p. 760. 



