360 PATHOGENIC BACTERIA. 



In gelatin puncture-cultures the growth is even more 

 characteristic than are the colonies. The bacilli begin 

 to grow along the entire track of the wire, most luxuri- 

 antly at the surface, where oxygen is plentiful. As the 

 growth progresses fine filaments like bristles, extend 

 from the puncture into the neighboring gelatin giving 

 the growth somewhat the appearance of an evergreen 

 tree inverted (Fig. 104). 



FIG. 104. Bacillus anthracis : gelatin puncture-culture seven days old 

 (Gunther). 



The more superficial of these threads reach about half- 

 way to the sides of the tube, while the deeper ones are 

 shorter and shorter, until near the apex branches cease. 

 When the projections are pretty well developed a distinct 

 surface-growth will be discerned, and if the tube be tilted, 

 one can observe that the gelatin beneath it has liquefied. 

 As the growth becomes older the liquefaction increases, 

 until ultimately the entire gelatin is fluid and the growth 

 is precipitated. 



Upon agar-agar the characteristics are few. The 

 growth takes place all along the line of inoculation as 

 a slightly translucent, slightly wrinkled layer with irreg- 

 ular edges, from which sufficient bacillary threads pro- 

 ject to give it a ciliated appearance to the naked eye. 

 When the culture is old the agar-agar turns a distinct 

 brown. Spore-formation is luxuriant upon agar-agar. 



