STREAKED PLATES. 



35 



Again, we not only have colonies developing from organisms which 

 have been fixed at the surface as the agar solidified in the plate, but 

 more numerous ones developing from bacteria caught in the depths 



FIG. 8. Petri Agar Plate. Made by spreading scrapings from the mouth over 

 sterilized nutrient agar; after 48 hours in the thermostat the light "colonies" 

 develop. Streaked plate. (Delafield and Prudden.) 



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of the media. Therefore we have superficial and deep colonies. 

 Except to the person of great experience, all deep colonies look alike 

 and there is at times great difficulty in deciding whether a colony is 

 deep or superficial. It is in the matter of trying to obtain information 



