210 BACTERIOLOGY. 



of endogenous spores, but when the spores are com- 

 pletely 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 of bacteria. 



HANGING-BLOCK CULTURES. Hill 1 has devised a 

 method for observing the development of individual 

 bacteria, which consists in the substitution for the ordi- 

 nary " hanging drop " of liquid or jelly a cube of solid- 

 ified 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 sur- 

 face. Place it, under surface down, on a slide and 

 protect it froni 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 forma- 

 tion 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 

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



