CULTURES, AND THEIR STUDY. 



207 



the tube, held almost horizontally, is rolled in this until 

 the entire surface of the glass is covered with a thin 

 layer of the solid medium (Fig. 27). Thus the tube 

 becomes the plate upon which the colonies develop. 



Fig. 27. — Esmarch tube on block of ice (redrawn after Abbott). 



Several little points need to be observed in carrying 

 out Esmarch' s method. The tube must not contain too 

 much culture-medium, or it cannot be rolled into an even 

 layer. In rolling the contents should not touch the cotton 

 plug, lest it be glued to the glass and its subsequent use- 

 fulness be injured. No water must be admitted from the 

 melted ice. 



The offspring of each bacterium growing upon the 

 film of gelatin constituting a plate-culture form a mass 

 which has already been pointed out as a colony. These 

 small bacterial families may be seen through a micro- 

 scope when still much too small for detection by the 

 naked eye, and because of their minuteness should always 

 be studied with the microscope. 



The original plates of Koch are very inconvenient for 

 such examination, because it is impossible to remove 

 them from the moist chamber and lay them upon the 

 stage of the microscope without exposing them to the 

 danger of contamination by the atmosphere, so that the 

 advantages of Petri dishes and Esmarch tubes, where 

 the examination may be made through the glass tube or 



