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 Pig. 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 3l 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 middle in a flame. After plugging with 

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

 The capillary tip on each tube should not 

 exceed 7 oi; 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- 

 ing blood to obtain sterile ^meter in order to obtain the best yield of 



serum. '' 



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 



