ISOLATION OF BACTERIA IN PURE CULTURE 179 



that even the separate colonies contain both organisms. 

 This is not common, however. The plate colonies frequently 



Fig. 129. — Dilution plates. 



shows the first dilution, the colo- 



nies are so numerous and small that they are invisible (compare Fig. 130) ; 

 2, shows fewer and heiice Jarger colonies, but too crowded to isolate (pompare 

 Fig. 131); 3, shows. the colonies, larger and well separated, so that it is easy 

 to isolate from thein (compare Fig. 132). 



develop from groups of bacteria which were not separated, 

 but as these are of the same kind the culture is essentially 

 pure. 



Fig. 130. — A portion of plate 1 in Fig. 129 as seen under the low-power 

 objective. X 100. Very small, closely crowded colonies. 



Another method which is frequently applicable with 

 material from human or animal sources is to (3) rub the 

 material over the surface of a slope tube or of medium solid- 

 ified in a Petri dish with a sterile heavy platinum needle, 



