108 TEXT-BOOK OP BACTERIOLOGY. 



manipulation. This fishing is, however, not very easy to learn. 

 We must avoid touching other parts of the gelatin before and after 

 the stroke; we must strike but one colony, and that with a sure 

 and steady hand, and. this requires an amount of dexterity that 

 practice alone can give. The colonies which lie deep in the interior 

 of the gelatin often tax the skill of the beginner severely, and put 

 his patience to a hard test. 



The process is, however, still more diflBcult when we have to deal 

 not with simple plates, but with dish or test-tube preparations. 

 With the former, the high edge is a decided hindrance for the 

 motions of the needle; with the latter, the cotton plug must first 

 be removed while the tube already lies under the microscope, then 

 the platinum wire must be carefully introduced and advanced to 

 the part where the colony in question is situated, after which comes 

 the fishing. It is scarcely possible to avoid touching other portions 

 of the food medium with some part or other of the instrument. 



If one has a portion of a colony on the point of the needle, it is 

 transplanted into solid gelatin, taking off the cotton plug from a 

 test-tube and sticking the needle deeply and vertically into the 

 gelatin. It is then withdrawn again quickly, the plug is replaced, 

 and the needle-point culture may now be left to develop. The ob- 

 ject here being to plant an absolutely pure culture of a certain 

 species of bacteria, special pains should be taken to prevent all for- 

 eign pollutions. We therefore endeavor, when we open the tube, 

 to hold the mouth of it in such a manner that no germs floating in 

 the air may fall into it. In some cases it is even better to hold the 

 tube inverted, with the opening directed straight downward, and 

 press it down from above upon the point of the upright needle. 

 Of course the cotton plugs should not, in the mean time, be laid on 

 the table, but carefully held between two fingers of the left hand. 



When the needle-point culture is planted, we return to the 

 mother colony and employ the remainder of it for a hanging drop 

 or a stained preparation. In this way we obtain specimens of the 

 species which have just been transplanted, which may be afterward 

 useful for comparison. 



In about the same time which was required for the formation of 

 distinctly visible colonies on the glass plates, a culture makes its 

 appearance along the needle hole in the test-tube. The spread of 

 growth is, indeed, very different with different species, and as a 

 rule all the features which were recognized in the glass-plate cul- 

 ture present themselves here again, with but little alteration. 



If the appearance of the micro-organisms in the needle-point 

 culture is not quite so characteristic as on the glass plate, the rea- 



