INTERLAMELLAR FILMS 131 



misled by a flotation of the cells due to currents set up 

 in the fluid from some cause or other all the particles 

 then tending to move in the same direction. 



Another purpose for which the hanging-drop cultivation 

 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 of these cultivations 

 be made, say twenty, in an exactly similar manner, 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. 1 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 equilateral 

 triangle, the side of which measures about one 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 pre- 

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

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

 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 



1 Delepine, Lancet, 1891, vol. i, June 13. 



