On the Cultivation of Bacteria. By Edgar M. CrookshanJc. 27 



during the few moments it is exposed to the air, it grows exactly 

 upon the spot upon which it fell, and can be easily recognized as a 

 stranger. 



To maintain the colonies isolated from one another during their 

 growth, and free fi'om contamination, it is only necessary to thin 

 out the micro-organisms sufficiently, and to limit the exposure of 

 the plates to the air to as short a time as possible, both during 

 their preparation and during their subsequent examination. 



The result will depend upon the way in which the thinning or 

 fractional cultivation has been carried out, and the colonies will be 

 found to develope in the course of a day or two, the time varying 

 with the rapidity of growth of the micro-organism and the tem- 

 perature of the room. 



If we have prepared three plates, we shall commonly find that 

 the lower plate will contain a countless number of colonies which, 

 if the micro-organism liquefies gelatin, speedily commingle and 

 produce in a very short time a complete liquefaction of the whole 

 of the nutrient medium. In the middle plate or "the first 

 thinning," the colonies will also be very numerous ; while in the 

 uppermost plate, " the second thinning," the colonies are com- 

 pletely isolated from one another, with an appreciable surface of 

 gelatin intervening. The microscopical appearances of the colonies 

 can perhaps best be observed by placing the plate upon a slab of 

 blackened plate glass, or upon a porcelain slab if the colonies are 

 coloured. 



The microscopical appearances are examined by placing a 

 selected plate upon the stage of the Microscope, and it is better to 

 have a larger stage than usual for this purpose. The smallest 

 diaphragm is employed, and the appearances studied principally 

 with a low power. 



The morphological characteristics of the micro-organisms of 

 which the colony is formed can then be examined in the following 

 way. A platinum needle bent at the extremity into a miniature 

 hook is held like a pen, and the hand steadied by resting the little 

 finger on the stage of the Microscope. The extremity of the 

 needle is steadily directed between the lens and the gelatin with 

 out touching the latter, until on looking through the Microscope 

 it can be seen in the field above, or by the side of the colony 

 under examination. The needle is then dipped into the colony, 

 steadily raised, and withdrawn. V^'ithout removing the eye from 

 the Microscope, this small operation may be seen to be successful, 

 by the colony being disorganized or completely removed from the 

 gelatin. It is, however, not easy to be successful at first, but with 

 practice this can be accomplished with rapidity and precision. A 

 cover-glass preparation is then made in the usual manner, by 

 rubbing the extremity of the platinum needle in a droplet of 



