CULTURE METHODS AND APPARATUS 91 



condensation which gathers on the medium permits certain 

 bacteria to overgrow the others and cause confusion. 



In the plate method the single bacteria are separated and 

 imbedded in the solid medium which is suitable for their 

 growth, and they soon produce, as a result, a growth or 

 colony varying in size from a pinhead to a centimeter, 

 according to the nature of the bacterium and the conditions 

 under which it is grown. It is taken for granted in this 

 method that each colony grows from a single individual, 

 originally imbedded in the medium. This may or may not 

 be true. Among the bacteria it generally is true, but several 

 yeasts may cling together and form a colony, the individuals 

 of which are of more than one species or variety. To over- 

 come this difficulty Hanson introduced his pure culture 

 method, which consists of a gelatin or agar plate made on a 

 cover-glass which can be placed under the microscope and 

 examined. The cover-glass is marked off into squares which 

 are numbered, and a record is made of the cells occurring in 

 the various squares, and only those colonies are used for 

 subculturing which are derived from a single cell. 



Subcultures. Test tube cultures are usually used in mak- 

 ing subcultures because of the convenience in handling and 

 also because of the slight danger from contamination. The 

 gelatin stab culture is frequently used. In this, the platinum 

 needle w r hich has been charged with the particular bacteria is 

 plunged to the depth of the gelatin in the test tube, so that 

 the bacteria are left all along the line of the stab. These cul- 



