2 5 



Cultures, and their Study 



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, inverted, so that the 

 water of condensation may not drop from the lid upon the 

 culture film and spoil the cultures, and stood away for the 

 colonies to develop. 



To overcome the difficulty of excessive water of conden- 

 sation Hill has introduced lids made of porous clay, by which 

 the moisture is absorbed. These can be obtained from 

 most laboratory purveyors. 



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 

 remove them from the sterile chamber to the stage of a 

 microscope and so expose them to the air, and to con- 

 tamination, but to examine colonies in the Petri dish, one 

 simply 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. 56. 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- 



