32 ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 
When thoroughly solidified, invert the plates and incubate at 37° C. for 
two days. 
It will simplify matters to arrange your Petri plates and water blanks at 
the beginning. Mark the plates and water blanks with their proper dilution 
figures. Also label the Petri dishes with such other marks of identification as 
may be needed. Make the dilutions desired in the manner outlined in the dia- 
gram given below. Be sure to change pipettes with each dilution bottle. On the 
diagram, the transfers which may be made with the same pipette are marked 
with the same letter. 
