THE STUDY OF COLONIES 153 



little pellicle of raw cotton. All these differences, and many 

 more, aid us in saying that these objects must be different 

 in their constitution. With a pointed platinum needle take 

 up a bit of one of these small islands, prepare it for micro- 

 scopic examination (see chapter on Stained Cover-slip 

 Preparations), and examine it under the high-power oil- 

 immersion objective, with access of the greatest amount of 

 light afforded by the illuminator of the microscope. The 

 preparation will be seen to be made up entirely of bodies 

 of the same shape; they will all be spheres, or ovals, or 

 rods, but not a mixture of these forms, if proper care in 

 the manipulation had been taken. Examine in the same 

 way a neighboring spot which possesses different naked-eye 

 appearances, and often it will be found to consist of bodies 

 of an entirely different appearance from those seen in the 

 first preparation. 



These spots or islands on the surface of the plates are 

 colonies of bacteria, differing severally, not only in their 

 gross appearances, the one from the other, but, as our cover- 

 slip preparations show, in the morphological characteristics 

 of the individual organisms composing them. 



If from one of these colonies a second set of plates be 

 prepared, the peculiarities which were first observed in it 

 will be reproduced in all of the new colonies which develop; 

 each will be found to consist of the same organisms as the 

 colony from which the plates were made. In other words, 

 these peculiarities are constant under uniform conditions. 



The appearance of the colonies developing from all organ- 

 isms is regulated by their location in the medium in which 

 they are growing. When deep down in the medium they are 

 usually round, oval, or lozenge-shaped; whereas when on 

 the surface of the gelatin or agar they may take quite a 



