196 BACTERIOLOGY 



Hanging-block Cultures.— Hilli has devised a method for 

 observing the development of individual bacteria, which 

 consists in the substitution for the ordinary "hanging drop" 

 of liquid or jelly a cube of solidified agar-agar, on the surface 

 of which the bacteria are distributed. 



The "hanging block" is prepared as follows: "Pour 

 melted nutrient agar into a Petri dish to the depth of one- 

 eighth to one-quarter inch. Cool this agar and cut from it 

 a block about one-quarter to one-third inch square and of 

 the thickness of the layer of agar in the dish. This block has 

 a smooth upper and under surface. Place it, under surface 

 down, on a slide and protect it from dust. Prepare an 

 emulsion in sterile water of the organism to be examined if 

 it has been grown on a solid medium, or use a broth culture; 

 spread the emulsion or broth upon the upper surface of the 

 block, as if making an ordinary cover-slip preparation. 

 Keep the shde and block in an incubator at 37° C. for five 

 to ten minutes to dry slightly. Then lay a clean sterile 

 cover-slip on the inoculated surface of the block in close 

 contact with it, avoiding, if possible, the formation of air- 

 bubbles. Remove the slide from the lower surface of the 

 block, and invert the cover-slip so that the agar-block is 

 uppermost. With a platinum loop run a drop or two of 

 melted agar along each side of the agar block where it is 

 in contact with the cover-slip. This seal hardens at once, 

 preventing slipping of the block. Place the preparation in 

 the incubator again for five or ten minutes to dry the agar 

 seal. Invert this preparation over a moist chamber and seal 

 the cover-slip in place with white wax or parafiin. Vaselin 

 softens too readily at 37° C, allowing shifting of the cover- 

 slip. The preparation may then be examined at leisure." 



' Journal of Medical Research, 1902, vol. vii, p. 202. 



