104 TEXT-BOOK OF BACTERIOLOGY. 



Within the mass of the gelatin the colony, in forming, has to 

 contend against the considerable resistance of its solid surround- 

 ings; it must conquer the ground step bj- step; besides which it 

 often has to suffer from want of sufficient oxygen to enable it to 

 thrive vigorously. On the surface the case is different: there no 

 resistance checks its spread, and the conditions are doubtless mc/e 

 favorable to its development. 



Thus many of the most distinguishing marks by which the dif- 

 ferent species of bacteria are recognized, in particular the liquefac- 

 tion of gelatin and the formation of pigment, are only seen to their 

 full extent in the surface colonies of a plate, and these are, conse- 

 quently, of special value to us in forming definite conclusions. 

 When more positive information is necessary as to the construc- 

 tion and composition of a colony, there are two methods of pro- 

 ceeding. Lay a cover-glass over the colony in question, press it 

 down a little on the gelatin, then remove and mount in water on a 

 slide. Place a drop of oil on the cover-glass and examine at once 

 with the immersion lens. As it is an unstained preparation, emploj' 

 the diaphragm, which, for thv. high magnifying power required, 

 should have about the same aperture as was recommended for the 

 examination of objects in the hollowed slide. At the edge of the 

 colony in particular we may often make verj'' instructive observa- 

 tions; the bacteria here lie somewhat freer — one can distinguish in- 

 dividual cells and can clearly mark how thej' multiply and spread 

 further and further over the solid food medium. 



In consequence of the small focal length possessed by our im- 

 mersion systems, this kind of observation is hardly possible with 

 any but surface colonies. These latter, alone and exclusively, can 

 be examined by another somewhat similar process, but which re- 

 quires the aid of staining to show the finer structure of the colonies. 



Take a thin cover-glass, lay it on the gelatin plate, press it 

 lightly upon the surface colony, and then lift it carefully up again 

 with the forceps. An exact print of the colony sticking to the 

 cover-glass will be obtained, which can be stained in the usual 

 manner and then examined. Allow it (the print preparation) to 

 become air-dry, pass it three times slowly through the flame, give 

 it a drop of fuchsin, or gentian-violet, decolorize with water, and 

 submit it to microscopic examination. 



If the colonies are not too far advanced in their development, 

 and the gelatin is only just beginning to liquefy, we obtain on 

 plates of twent3'-four — or at the most thirty-six — hours' standing 

 jjrints of remarkable delicacy, 3'et of perfect sharpness. Even with 

 a low power the colonies can be seen in their characteristic size 



