MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 29 



inoculated flasks are vigorously shaken, and then left in repose, 

 the individual cells will sink to the bottom, and become 

 deposited on the wall of the flask. It is self-evident that if 

 the flask contains, for instance, three cells, these cells will 

 always, or at least in the majority of cases, be deposited 

 separate from each other and apart, on the bottom. After 

 some days, if the flask is raised carefully, it will be observed 

 that one or more white specks have formed on the bottom of 

 the flask. If only one such speck be found, we have obtained 

 a pure culture. 



It is evident that by means of this method we are also 

 able to introduce a single cell into the flask with nutritive 

 solution. 



It was by this method that Hansen prepared all his first 

 pure cultures, with which he carried out his fundamental 

 researches on alcoholic ferments. 



Solid nutrient media have also been employed for the 

 preparation of pure cultures for use in physiological investi- 

 gations. The foundation of such methods was laid by Schroeter 

 (1872), who, in his researches on pigment-bacteria, employed 

 slices of potatoes among other nutrients. He had observed 

 that when such slices had been exposed for some time to the air, 

 specks or drops of different form and colour made their appear- 

 ance. Each of these specks contained most frequently one 

 species of micro-organism. 



R. Koch subsequently considerably improved this method. 

 He at first prepared his pure cultures by means of streak 

 growths in nutritive gelatine. Afterwards he devised a far 

 better method, the so-called plate-culture method (1883). 

 He proceeds in the following manner. A trace of the crude 

 culture is transferred to a large proportion of sterilised water. 

 From this a small quantity is transferred to a flask containing, 

 for instance, a mixture of meat-broth and gelatine warmed to 

 30 C. The flask is shaken in order to distribute the germs, 

 and the contents poured on to a large glass plate, which is then 

 covered with a bell-glass. The gelatine quickly sets and the 



