A YEAR IN SCIENCE 99 



(if properly done) and is ready for inoculation. The 

 agar medium is prepared in the same way, except that 

 instead of using 20 grams of gelatin, use 2/5 of a gram 

 of agar. In all other respects the culture is made pre- 

 cisely as the gelatin culture medium. Potato may be 

 used by paring and placing thin cut slices in the 

 sterilized dishes. Sterilize as indicated above. 



1. After the medium has hardened, expose the 

 medium of one dish to the air of the laboratory for 

 three minutes. Cover the dish, label it No. 1, and set 

 aside until the next recitation period. 



2. At the same time expose the medium of another 

 dish to the air out of doors for three minutes. Cover 

 and label No. 2, and set aside. 



3. Touch the surface of the medium of a third dish 

 with the fingers in several places. Cover, label No. 3, 

 and set aside. 



4. Over the surface of the medium of a fourth dish, 

 roll the end of your lead pencil (which you so fre- 

 quently place in your mouth). Cover, label No. 4, 

 and set aside. 



5. If a fly is obtainable, capture it, and place it 

 inside another dish, permitting it to walk over the 

 surface of the medium. Free the fly, cover the dish 

 and label No. 5. Set aside until next recitation. 



(>. Set one dish aside unopened as a check, and label 

 No. 6. 



Place all vessels under similar conditions of light, 

 heat, etc. Make daily observations for the appearance 



