THE OPSONIC INDEX 



231 



ing which time the blood coagulates and is spontane- 

 ously drawn toward the sealed end, the curved or 

 inlet end may also be sealed. The serum begins to lose 

 its opsonic content in six hours and should not be 

 utilized for index determination after this time. Ten 

 to fifteen drops of normal blood 

 are then allowed to fall from the 

 finger-tip into a test-tube of normal 

 salt solution containing 1.5 per cent, 

 sodium citrate (Fig. 23). The 

 citrate will prevent the clotting of the 

 blood, and the corpuscles are thor- 

 oughly washed by centrifugation two 

 or three times in normal saline (Fig. 

 25 ) , to render them neutral and abso- 

 lutely free from serum. The white 

 blood-corpuscles, being lighter than 

 the red, will overlie the latter as a 

 grayish layer of " leucocytic cream " 

 (Fig. 24), whereupon they may be 

 picked up by a pipette and trans- 

 ferred to another receptacle (Fig. 

 A six- to twenty-four-hour culture of the 

 given bacterium is washed off in normal salt solu- 

 tion (Fig. 27), then centrifuged to throw down 

 all bacterial clumps. The supernatant suspension 

 should contain only individual bacteria in not too 



1/ 



Fio. 22. 



-Capillary 

 glass capsules for the 

 collection of specimens 

 of blood. A, simple 

 straight; B, Wright's. 



26 



