76 



CULTURES IX SOLID MEDIA. 



without the exposure which occurs when a plate prepared by Koch's 

 method is under examination. 



In agar-agar cultures or in gelatin cultures of non-liquefying 

 bacteria made in Petri's dishes, we may examine and count colonies, 

 without removing the cover, by inverting the dish. 



In pouring the liquefied gelatin from the test tubes in which the 

 dilution has been made into sterilized Petri's dishes, care must be 

 taken to first sterilize the lip of the test tube by passing it through 

 the flame of a lamp. We may at the same time burn off the top of 

 the cotton plug, then remove the remaining portion with forceps, 

 when the lip has cooled, for the purpose of pouring the liquid into the 

 shallow dish. 



Von Esmarch' s Roll Tubes. Another very useful modification 

 of Koch's plate method is that of von Esmarch. Instead of pouring 

 the liquefied gelatin or agar medium upon plates or in shallow 



FIG. 45. 



dishes, it is distributed in a thin layer upon the walls of the test tube 

 containing it. This is done by rotating the tube upon a block of ice 

 or in iced water. Esmarch first used a tray containing iced water, 

 and to prevent the wetting of the cotton filter a cap of thin rubber 

 was placed over the end of the tube. It is more convenient to turn 

 the tubes upon a block of ice having a horizontal flat surface, in 

 which a shallow groove is first made by means of a test tube con- 

 taining hot water (Fig. 45). Or, in the winter, we may turn the 

 tube under a stream of cold water from the city supply i. e. , from a 

 faucet in the laboratory. A little practice will enable the student to 

 distribute the culture medium in a uniform layer on the walls of the 

 test tube, and as soon as it is quite solidified these may be placed 

 aside for the development of colonies from the bacteria which had 

 been introduced. When roll tubes are made from the agar jelly it is 

 best to place the tubes in a nearly horizontal position, for if placed 

 upright at once the film of jelly is likely to slip from the walls of the 



