2l8 



Cultures, and their Study 



lation the cotton plugs are pushed into the tubes until even 

 with their mouths, and then covered with a rubber cap, which 

 protects them from wetting. A groove is next cut in a block 

 of ice, and the tube, held almost horizontally, is rolled in this 

 until the entire surface of the glass is covered with a thin 

 layer of the solidified medium (Fig. 45). Thus the tube itself 

 becomes the plate upon which the colonies develop. 



In carrying out Esmarch's method, the tube must not con- 

 tain too much of the culture medium, or it cannot be rolled 

 into an even layer ; the contents should not touch the cotton 

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

 ness injured, and no water must be admitted from the melted 

 ice. 



Colonies. The progeny of each bacterium form a mass 



Fig. 46. Types of colonies: a, Cochleate (B. coli, abnormal form); 

 b, conglomerate (B. zopfii) ; c, ameboid (B. vulgatus) ; d, filamentous 

 (Frost). 



Fig. 47- Surface elevations of growths: a, Flat; b, raised; c, convex; 

 d, pulvinate; e, capitate; /, umbilicate; g, umbonate (Frost). 



which is known as a colony. When these are separated from 

 one another, each is spoken of as a single colony, and dif- 

 ferent characteristics belonging to different micro-organisms 

 enable us at times to recognize by macroscopic and micro- 

 scopic study of the colony the particular kind of micro- 

 organism from which it has grown. The foregoing illustra- 

 tions show the various types of colonies and the legends 

 the terms used in describing them (Fig. 46). 



Growing colonies should be observed from day to day, as 

 it not infrequently happens that unexpected changes, such 

 as pigmentation and liquefaction, develop after the colony 



