MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 31 



potato 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 appearance. 

 Each of these specks contained most frequently one species of 

 micro-organism. 



KOCH considerably improved this method. He at first pre- 

 pared 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 con- 

 tents poured on to a large glass plate, which is then covered 

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

 remain enclosed in the solid mass. In a few days they develop 

 to colonies points or specks which are visible to the naked 

 eye. The purity of the specks of bacteria in the gelatine is- 

 ascertained, according to KOCH, partly by their appearance, 

 colour, form, etc. 



When regarded more closely it will be seen, however, that 

 there is no essential difference between this distribution of 

 the germs in the liquid gelatine, and the former dilution by 

 means of liquids. The same uncertainty is always present : 

 neither the macroscopical observation of the appearance of the 

 colony nor the microscopical examination of its contents gives 

 any surety of its only containing one species. 



The only possibility of securing a really pure culture in the 

 gelatine consists in the direct observation of one individual 

 germ and its development. 



HANSEN has done this in the case of yeast-cells, and the 

 method which he contrived for the purpose is as follows. The 

 layer of gelatine formed by the solidified nutritive liquid is 

 arranged in such a way that the position of the isolated germs 

 can be observed under the microscope. The position of these 

 germs, then, is accurately marked, and the cell can be seen to 

 develop and propagate step by step. 



For the glass-plate is substituted a round cover-glass of 



