70 CULTURES IN SOLID MEDIA. 
always to hold the tube inverted during the inoculation, and not to 
remove the cotton air filter until we are ready to make it. The 
cotton plug is then returned to its place and the platinum needle 
again brought to a red heat to destroy any bacteria which remain 
attached to it. 
Sometimes it is an advantage to have the culture medium with a 
Fia. 40. 
sloping surface, as shown in Fig. 40, We may then draw the nee- 
dle over the surface in a longitudinal direction, and by this means 
distribute the seed in a line along which development will take place. 
The characters of growth in these stab cultures in gelatin are 
Fria, 40a. 
very various. Non-liquefying bacteria may grow only on the sur- 
face, as at a, Fig. 404; or both on the surface and along the line 
of puncture, as at b; or only at the bottom, as atc. In the first 
case the microdrganism is aérobic—that is, it requires oxygen, and 
grows only in the presence of this gas. In the second case it is 
not strictly aérobic, but may grow either in the presence of oxygen 
