THE USE OF THE ORDINARY CULTURE MEDIA 53 



In filling tubes, care must be taken to run the liquid down 

 the centre, so that none of it drops on the inside of the upper 

 part of the tube with which the cotton-wool plug will be in 

 contact, otherwise the. latter will subsequently stick to 

 the glass and its removal will be difficult. In the case of 

 liquid media, test-tubes are filled about one-third full. With 

 the solid media the amount varies. In the case of gelatin 



A e 



FIG. 13. Tubes of media. 



The apparatus explains itself. 

 The indiarubber stopper with 

 its tubes ought to be steril- 

 ised before use. 



media, tubes filled one-third full and allowed to solidify 

 while standing upright, are those commonly used. With 

 organisms needing an abundant supply of oxygen the best 

 growth takes place on the surface of the medium, and for 

 practical purposes the surface ought thus to be as large as 

 possible. To this <end " sloped " agar and gelatin tubes are 

 used. To prepare these, tubes are filled only about one-sixth 

 full, and after sterilisation are allowed to solidify, lying on their 

 sides with their necks supported so that the contents extend 



