130 BIOLOGY AND TECHNIQUE 



be adjusted to a neutral reaction. It is used for stab cultures and is 

 designed chiefly for pneumococcus cultivation and storage. 



Dorsett Egg Medium This medium is chiefly useful for the cul- 

 tivation of tubercle bacilli. 



1. Carefully break eggs and drop the contents into a wide-mouthed 

 flask. Break up the yolk with a sterile platinum wire, and shake up 

 the flask until the whites and yolks are thoroughly mixed. 



2. Add 25 c.c. of distilled water to every four eggs; strain through 

 sterile cloth. 



3. Pour 10 c.c. each into sterile test tubes and slant in an inspissator 

 and expose to 73 C. for four to five hours on two days. 



4. On the third day, raise the temperature to 76 C. 



5. The sterilization may be finished by a single exposure to 100 

 C. in the Arnold sterilizer for fifteen minutes. Before inoculation, add 

 two or three drops of sterile water to each tube. 



For a description of Petroff s medium for the isolation of tubercle 

 bacilli, see page 484. 



Potato Media. Large potatoes are selected, washed in hot water, 

 and scrubbed with a brush. They are peeled, considerably more than 

 the cuticle being removed The peeled potatoes are washed in running 

 water, following which cylindrical pieces are removed with a large 

 apple corer. The cylinders are cut into wedges. 



Since the reaction of the potato is normally acid, this should be cor- 

 rected by washing the pieces in running water over night, or, better, 

 by immersing them in a one per cent solution of sodium carbonate for half 

 an hour. 



The pieces are then inserted into the large variety of test tubes 

 known as "potato tubes." (See Fig. 21, c.) In the bottom of the 

 tubes a small amount of water (about 1 c.c.) or a small quantity of 

 moist absorbent cotton should be placed in order to retard drying out 

 of the potato. The tubes are sterilized by fractional sterilization, 

 twenty minutes to half an hour in the Arnold sterilizer on three suc- 

 cessive days. 



Glycerin Potato. In preparing glycerin potato the potato wedges 

 are treated as above, and are then soaked in a ten to twenty-five per 

 cent aqueous glycerin solution for one to three hours. A small quantity 

 of a ten per cent glycerin solution should be left in the tubes. In steril- 

 izing these tubes, thirty minutes a day in the Arnold after heating of 

 the sterilizer will sterilize without altering the glycerin. 



Milk Media. Fresh milk is procured and is heated in a flask for 



