MICROSCOPIC EXAMINATION OF PREPARATIONS 213 



have to oxygen, for, their growth being on the surface, they 

 are surrounded by this gas unless means are taken to prevent 

 it. By the hanging-drop method, however, more than -this 

 specific property may be determined. It is possible not only 

 to detect the stages and steps in the formation of endogenous 

 spores, but when the spores are completely formed their 

 germination into mature rods may be seen by transferring 

 them to a fresh bouillon-drop or drop of agar-agar preserved 

 in the same way. The word rods is used because we have as 

 yet no evidence that, endogenous spore-formation occurs 

 in any of the other morphological groups or bacteria. 



Hanging-block Cultures. Hill 1 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" i's 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 slide 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- 



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



