TEXT-BOOK OF BACTERIOLOGY. 103 



large stage, and, on the other hand, the plates should not be too 

 large, but about 12 by 9 cm. 



Before commencing- a microscopic examination, notice the ap- 

 pearance of the plates with the naked eye. The original will, as a 

 rule, be so thickly covered with germs as to be almost worthless, 

 and therefore we generally employ the first or second dilution. 

 The plate is pushed slowly under the microscope, the focus being 

 moved up and down by the coarse adjustment so as to bring the 

 entire depth of the gelatin layer under observation. 



The extraordinary variety of appearance of the separate pure 

 cultures is very striking*. This variety appears still greater now 

 than it did with the naked eye, for we are now able to note the finer 

 distinctions in the forms of the colonies. Some are curiously gran- 

 ulated, others show concentric rings, many spread out quite evenly, 

 some with ribs, like leaves; some are curled, rolled up like a ball 

 of thread, or provided with tendrils; the majority are of a light 

 yellow or brownish color, and many are distinguished by some defi- 

 nite pigment. 



Yet it would be a mistake to consider two colonies as being of 

 different species merely because they present a different appear- 

 ance. A certain degree of caution is necessary, and no definite 

 judgment can be formed without repeated experiments. 



It is sometimes especially difficult to recognize colonies lying in 

 the mass of the gelatin as being identical with others of very dif- 

 ferent appearance on the surface of the food medium. For in- 

 stance, examine a plate containing colonies of typhus bacilli alone, 

 and it would hardly be supposed that the different colonies were 

 composed of the same micro-organism. In one place small, some- 

 what elliptical or whetstone-shaped, dark-brown, slightly-granu- 

 lated colonies are seen, and near them may be some yellowish- 

 white, almost transparent discs, spreading out like leaves and 

 marked with ribs, which have scarcely a trace of resemblance with 

 the others, but which, nevertheless, proceed from the same species 

 of bacteria. 



Conviction of this fact will be gained by noting several transi- 

 tion forms between the two sorts of colonies; then, too, it is seen, 

 by prolonged observation, that one kind gradually merges into 

 the other; further, it will be found that a microscopic examination 

 shows everywhere the same rod-cells; and lastly, it will be found 

 that whether new plates are prepared with one or the other kind 

 of colony, both forms will always be developed over again. 



This difference in the forms of the colonies may be easily ex- 

 plained. 



