PREPARATION OF PURE CULTURE. 51 



the germs in liquefied gelatine, and Lister's method of 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 did this for yeast by using Bottcher's moist 

 chamber. The lower side of the cover-glass is covered with a 

 layer of wort-gelatine, in which the yeast cells are distributed. 

 On account of the size of the latter, it is possible to see whether 

 a single cell lies so wide apart from other cells that the colony 

 developed from it will form a pure culture. 



The chamber is then either allowed to remain under the 



Fig. 12. Jorgensen's moist chamber with etched squares and numbers. 



microscope, in order that the propagation of the germs may 

 be directly followed, or the positions of well isolated germs 

 are marked, either by dividing the glass-cover into small 

 squares, or by means of the object marker, and the apparatus 

 is placed in the incubator until the colonies are fully developed. 

 The cover-glass is then lifted off and placed under a bell-jar, 

 so that the gelatine layer is turned upwards, and the colonies 

 are transferred into flasks. In the author's laboratory moist 

 chambers like that represented in Fig. 12 are used, the cover- 

 glass being etched with 16 squares and numbers. The situa- 

 tion of the cells is then marked on a sketch plan, which shows 

 all the etched numbers and squares. The author has altered 

 the process by cementing the cover-glass on to the glass ring, 

 and fastening the latter to the object glass with vaseline. 

 To remove the chamber, the ring is lifted off, and this is a 



