FISHING COLONIES 253 



the sterilised loop to a fresh gelatine culture tube, and 

 after incubation the growth in each subculture would 

 correspond culturally and microscopically with that of 

 the plate colony from which it was derived, the object 

 aimed at would therefore be achieved. 



Usually, however, the colonies cannot be thus readily 

 differentiated, and unless they are "worked up" in an 

 orderly and systematic manner much labour will be 

 vainly expended and valuable time wasted. The fol- 

 lowing method minimises the difficulties involved. 



(A) Inspection. 



a. Without opening the plate carefully study the 

 various colonies with the naked eye, with the assistance 

 of a watchmaker's lens or by inverting the plate on 

 the stage of the microscope and viewing with the 

 i -inch objective through the bottom of the plate and 

 the layer of medium. 



b. If gross differences can be detected mark a small 

 circle on the bottom of the plate around the site of 

 each of the selected colonies, with the grease pencil. 



c. If no obvious differences can be made out choose 

 nine colonies haphazard and indicate their positions by 

 pencil marks on the bottom of the plate. 



(B) Fishing Colonies. 



a. Take a sterile Petri dish and invert it upon the 

 laboratory bench. Rule two parallel lines on the 

 bottom of the dish with a grease pencil, and two more 

 parallel lines at right angles to the first pair so dividing 

 the area of the dish into nine portions. Number the 

 top right-hand portion i, and the central bottom por- 

 tion 8 (Fig. 139). Revert the dish. The numbers i 

 and 8 can be readily recognised through the glass and 

 by their positions enable any of the other divisions to 

 be localised by number. This is the stock dish. 



b. Slightly raise the cover of the dish, and with a 



