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 1, and the central bottom portion 8 (Fig. 139). Revert the dish. The numbers 1 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 sterile teat-pipette deposit a small drop of sterile water in the centre of each of the nine divisions.