THE CULTIVATION OF BACTERIA. 93 



In cases where a very deep stab-culture is made in gelatin or agar, where the 

 growth appears in the lower part of the tube by preference, it is supposed to be 

 anaerobic. Koch covered part of the surface of a gelatin plate with a bit of 

 sterilized mica or a cover-glass; bacteria which grew beneath this place were 

 considered to be anaerobic. Another method was to cover the surface of the 

 gelatin in the culture-tube with sterilized oil. W. H. Park has recommended 

 a mixture of solid paraffin with 25 to 50 per cent, of fluid paraffin or albolene 

 as a covering for the surface of anaerobic cultures. This mixture has a semi- 



FIG. 31. Streak culture of the potato bacillus (natural size), showing an 

 aerobic organism which will not grow under a cover-glass. 



solid consistency, and does nor retract at the edges on cooling. The paraffin 

 prevents the absorption of oxygen except to a small extent at the edges. The 

 method is useful for large quantities of culture material, as in flasks. Esmarch 

 advised making roll-tubes, and after cooling them to fill them with a melted 

 gelatin cooled down to near the point of solidification. Hueppe made use of 

 eggs in their shells. The egg-shell was carefully cleaned, sterilized with a solu- 

 tion of bichloride of mercury, washed with sterilized water and wiped dry with 

 sterilized cotton'. The end of the egg-shell was punctured with a hot needle. 

 Through the opening thus made the inoculation was accomplished. The 

 opening was closed with collodion. 



