STAB AND SMEAR CULTURES. 103 



Cultures of this form are not only useful as a means 

 of preserving pure cultures of the different organisms 

 with which we may be working, but serve also to bring 

 out certain characteristics of different organisms when 

 grown in this way. 



If gelatin is employed and the organism which has 

 been introduced into it possesses the power of bringing 

 about liquefaction, this result is by no means of the same 

 appearance for all organisms. Some organisms cause a 

 liquefaction which spreads across the whole upper sur- 

 face of the gelatin and continues gradually downward ; 

 again it occurs in a funnel shape, the broad end of the 

 funnel being uppermost and the point downward, corre- 

 sponding to the track of the needle. At times a stock- 

 ing- or sac-formed liquefaction may be noticed. 



Obtain a number of organisms from different sources 

 in pure cultures by the method given. Plant them as 

 pure cultures, all at the same time, in gelatin — preferably 

 gelatin of the same making — retain them under the same 

 conditions of temperature, and note the finer differences 

 in the way in which liquefaction occurs. 



