30 MICRO-ORGANISMS AND FERMENTATION. 



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 indivi- 

 dual 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 

 about 30 mm. diameter. This is fastened to a glass-ring, 

 which again is cemented to a thicker glass, thus forming the 

 moist chamber previously described (Fig. 2), and which is 

 adapted to the purpose, and carries a layer of solid gelatine on 

 its inner upper surface. The essential point in Hanserts 

 method is, that the leading principle " the starting-point of 

 a pure culture must be a single cell " is consistently carried 

 out, which is not the case in Kochs method. The germs 

 must be so sparsely distributed that comparatively few are 

 present in the gelatine layer ; the chamber is then either 

 allowed to remain under the microscope, in order that the 

 multiplication of the germs may be directly followed, or the 

 positions of the well-isolated germs are marked, either by 



