PLATE METHOD OF KOCH. 55 



It was plain that the organisms were to be distin- 

 guished, the one from the other, only by the structure 

 and general appearance of the colonies growing from 

 them, for upon their morphology alone this is impossible. 



What means could be devised, then, for separating the 

 individual members of a mixture, in such a way that 

 they would remain in a fixed position, and be sufficiently 

 widely separated, the one from the other, as not to inter- 

 tere with the production of colonies of characteristic 

 appearance, which would, under the proper conditions, 

 develop from each individual cell ? 



If a test-tube of decomposed bouillon were poured out 

 upon a large flat surface, the individual bacteria in the 

 mass would be very much more widely separated the; 

 one from the other than they were when the bouillon 

 was in the tube. But they are in a fluid medium, and 

 there is no possibility of their either remaining separated 

 or of their forming colonies under these conditions, so 

 that it is impossible by this means to pick out the in- 

 dividuals in the mixture. 



If, however, it is possible to find some substance 

 which possesses the property of being at one time fluid 

 and at another time solid, which can be added to this 

 bouillon without in any way interfering with the life 

 functions of the bacteria, then, as solidification sets in, 

 the organisms will be fixed in their positions and the 

 conditions will be analogous to that seen on the bit of 

 potato. 



Gelatin possesses this property. At a temperature 

 which does not interfere with the life of the organisms 

 it is quite fluid, whereas when subjected to a lower 

 temperature it solidifies. When once solid it may be 



