HAGNING-DROP PREPARATION 147 



cause or other all the particles then tending to move in 

 the same direction. 



Another purpose for which the hanging-drop prepara- 

 tion may be employed is that of obtaining a permanent 

 record of the various phases through which an organism 

 may pass during its development. If a number, say 

 twenty, of hanging drops be made in an exactly similar 

 manner with nutrient broth, and afterwards kept under 

 identical conditions, and if at the end of every half hour 

 one of the preparations be taken, its cover-glass carefully 

 removed, and the droplet dried and stained, a permanent 

 record of the life-history of the organism is obtained 

 extending over ten hours. 



Various more elaborate forms of cells for hanging-drop 

 preparations can be obtained, some being provided with 

 inlet and exit tubes for the passage of various gases. For 

 anaerobic preparations cells are made having a groove 

 at the bottom into which a mixture of pyrogallic acid 

 and potash is introduced. 



The observation of hanging-drop cultivations at blood- 

 heat can be carried out on some form of warm stage. 



Interlamellar films. Another method of investigating the life- 

 history of organisms, especially moulds and protozoa, is by means 

 of interlamellar films. A glass slide 1^ by 3 in. is sterilised in the 

 Bunsen flame, and while hot three small drops of sealing-wax 

 are placed on it, so arranged that they form the apices of an equi- 

 lateral triangle, the side of which measures about 1 inch? and a 

 drop of sterile nutrient medium is deposited between them. A 

 cover-glass of about 1 in. in diameter is then sterilised in the 

 Bunsen flame, a droplet of a suitable nutrient medium is placed 

 upon it and inoculated with the organism to be observed, and the 

 prepared cover-glass is picked up with sterilised forceps, inverted, 

 and lowered on to the slide. The nutrient medium is thus con- 

 tained between the slide and the cover-glass, and by using a hot 

 wire, and so softening the sealing-wax, it can be spread out to 

 form as thin a layer as desired. The preparation is kept in a 



102 



