40 MICRO-ORGANISMS AND DISEASE. [CHAP. 



that in the first twenty-four or forty-eight hours not all the 

 different species of organisms sown out will have increased 

 equally in numbers in all tubes ; most probably only one 

 species in each tube, i.e. the one that grows best in this 

 particular medium and at this particular temperature, will be 

 found to have increased to an enormous extent, while the 

 others have made little or no progress as yet. The nourishing 

 fluid appears turbid, and filled chiefly with the one kind of 

 organism. Now draw out with a fresh capillary pipette a 

 minute droplet of this new culture and inoculate with a trace 

 of it a new culture-tube. The chances are that you inoculate 

 only one kind, that is, the one which is most abundant or 

 perhaps is solely present. After twenty-four hours' incubation 

 this new tube contains now probably only one kind of 

 organism. To make it quite certain, inoculate from this a 

 new culture-tube in the same manner, and now you probably 

 have sown only a single species. In this manner by 

 continued transferrence it is possible to obtain cultures of 

 only one species of organisms. Many conditions, such as 

 naked-eye appearances of a particular kind, coloration of the 

 culture-medium, formation of a pellicle, the quantity of 

 growth in a given time, soon indicate whether we have the 

 desired single species ; in some instances it is, however, 

 extremely difficult to isolate after this method. 



The method of " dilution " means diluting the culture-fluid 

 containing the various species to a very large extent with 

 some sterile indifferent fluid, such as well-boiled saline 

 solution of 0-6 per cent., and then inoculating new tubes 

 with a droplet of this greatly diluted material. For this 

 purpose draw into a rather large pipette a tiny droplet of the 

 old culture-fluid, then pass the pointed end of this pipette 

 into a test-tube or flask (plugged) containing well-boiled saline 

 solution, and draw up a quantity of this solution so as to 



