AGAR PLATES 



73 



liquid gelatine medium upon i^lates or in shallow dishes, it 

 is distributed in a thin layer upon the \yalls of a wide test- 

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

 ice or in iced water. It is more convenient to turn the 

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

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

 containing hot water. In the winter the tube can be re- 

 volved under the water service tap to solidif_y the jelly. 



A little practice will enable the operator to distribute the 

 jelly in an even layer on the walls of the tube, and as 

 soon as they are quite solidified they are set aside for the 

 colonies to develop. These tubes possess the advantage 



Fig. 16. — Method of making Esmarch's Roll Culture. 



that they are quickly made, they do not occupy much 

 room, and are well protected against atmospheric germs. 

 When the colonies have formed, they can readily be 

 counted and examined by means of a lens. 



Agar Plates. — The characters of the growth on nutrient 

 agar media are not so varied as in the case of those on 

 gelatine, and the plates are rather more difficult to manage ; 

 but this medium possesses the advantage of not liquefying 

 at 40° C, whereby the nature of the growth can be studied at 

 higher temperatures than is possible in the case of gelatine. 

 Agar media are not liquefied in the manner that gelatine 

 media are by many organisms. 



