XV. METHODS OF ISOLATION. 



THE work in the preceding sections, arranged to 

 demonstrate the chief biological characters of bacteria 

 in general, is intended to be carried out by means of 

 cultivations of various organisms previously isolated 

 and identified and supplied to the student in a state of 

 purity. A cultivation which comprises the prog- 

 eny of a single cell is termed a "pure culture"; one 

 which contains representatives of two or more species 



FIG. 138. Haematocytometer cell, showing, a, section through the centre of 

 the cell, and b, a magnified image of the cell rulings. 



of bacteria is spoken of as an "impure, " or "mixed" 

 "cultivation," and it now becomes necessary to indi- 

 cate the chief methods by which one or more organisms 

 may be isolated in a state of purity from a mixture; 

 whether that mixture exists as an impure laboratory 

 cultivation, or is contained in pus and other morbid 

 exudations, infected tissues, or water or food-stuffs. 



Before the introduction of solid media the only 

 method of obtaining pure cultivations was by "dilu- 

 tion" by no means a reliable method. "Dilution" 

 consisted in estimating approximately the number of 

 bacteria present in a given volume of fluid (by means 

 of a graduated-celled slide resembling a haemato- 



248 



