460 BACTERIOLOGY. 



similar to that used for sterilizing- plates. The larger 

 pipettes must be protected in a different way against sub- 

 sequent contamination from the air. The simplest pro- 

 cedure is to push a short cotton plug into the mouth end of 

 the pipette. The delivery end is inserted through a cotton 

 plug into a test-tube. The pipettes, thus protected, are 

 then sterilized in the usual way. 



Drawing of Blood, 



In studying agglutination and in many other experi- 

 ments it is necessary to draw a small quantity of blood from 

 an animal. When it is desired to obtain sterile 

 blood-serum the blood must be drawn under 

 strictly aseptic conditions. It should be re- 

 ceived in a sterile pipette similar to that shown 

 in Fig. 63. 



This pipette, which is a slight modification of 

 Nuttall's, can be readily prepared. A piece of glass 

 tubing- about 2.5 cm. in diameter and 50 cm. in length 

 is selected. A medium flame from a blast-lamp is 

 directed against the middle of the tube. As the 

 tube is slowly rotated and heated the glass softens 

 and a narrow constriction results. When constricted 

 to about one-third the original diameter, the two 

 halves are drawn apart so that the parallel tubes are 

 connected at an angle of about 120 by the resulting 

 capillary. They are finally separated by sealing the 

 latter at the mtddle in a flame. After plugging with 

 cotton, the pipettes are sterilized in a dry-heat oven. 

 The capillary tip on each tube should not 

 exceed 7 or 8 cm. Moreover, it is important 

 that the capillary should not be too narrow. 

 FIG. 63. Pipette for draw- The tip should be about 1.5 to 2 mm. in di- 

 ie?um* t0 btain Stedle ameter in order to obtain the best yield of 



, blood; 



In the absence of the wide glass-tubing indicated above, an ex- 

 cellent pipette can be made out of a large wide test-tube. The 



