25 



YJII. 



EXAMINATION OF COLONIES. 



In a greater or less period of time, varying usually 

 from 1 to 3 days, colonies develop on the plates, and as 

 soon as they become of suitable size they are ready for 

 examination. A careful study of the colonies on a plate 

 should first be made with the unaided eye. Several char- 

 acteristics of growth can often thus be recognized quite 

 early. Especial attention should be given to the form 

 and 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 kinds of colonies 

 which sometimes are quite different in appearance. Thus, 

 we may have surface colonies, and also deep colonies. 

 The former developing on the surface of the gelatin are 

 unhindered in their development, and may, therefore, 

 spread out and thus acquire peculiar characteristics, more- 

 over, 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 direc- 

 tions. The result is that deep colonies, as a rule, are 

 much alike in appearance. Thus, the form is usually 

 round or oval, with sharp edges, and the contents are 

 slightly granular and yellowish. 



The plate should now be placed upon the stage of the 

 microscope and the colonies carefully examined with a 

 low-power No. 3 objective. Further characteristics can 

 thus be brought out which have escaped the eye. 



The study of the micro-organisms which compose the 



