GELATIN AND POTATO MEDIA. 187 



Especial attention should be given to the form, and to 

 the appearance of the colonies; the presence, or absence of 

 pigment; liquefaction, or non-liquefaction of gelatin, etc. 

 It should be remembered, however, that a given organism 

 may give rise to at least two kind of colonies which some 

 times are quite different in appearance. Thus, we may 

 have surface as well as deep colonies. The former, develop- 

 ing on the surface of the gelatin, are unhindered in their 

 ■development, and may, therefore, spread out and thus 

 acquire peculiar characteristics. Moreover, having ready 

 access to oxygen, pigment formation and liquefaction will 

 be first seen in connection with these surface colonies. 

 'The deep colonies, on the other hand, are surrounded on all 

 sides by solid gelatin, and hence, much the same resistance 

 to growth will exist in all directions. The result is that 

 the deep colonies of various bacteria may be very much 

 alike in appearance. The deep colony is usually round or 

 oval, with sharp edges, and its contents are slightly granu- 

 lar and yellowish. 



In the study of the various bacteria, considerable 

 variety in the form of the surface and deep colonies will be 

 met with. In many cases the growth is so characteristic 

 that it is of great value in the recognition of that organism. 

 It should be remembered, however, that the cultural char- 

 acteristics of many bacteria are subject to more or less 

 variation, depending on the temperature at which the 

 growth occurs, as well as on the reaction and consistency 

 of the gelatin. The colonies on a hard, 15 per cent, gelatin 

 will be different, to some extent, from those that develop 

 on a soft, 10 per cent, gelatin. 



Plate 1 will usually have a cloudy or milky appearance. 

 On examination with a low power — No. 3 objective — the 

 cloudiness will be found to be due to countless numbers of 

 minute, round colonies. This will be the case when the 

 material which is employed for the inoculation is rich in 

 bacteria. Only exceptionally will plate 1 have a sufficiently 



