LKUCOCYTES EMULSION. 27 



packed together and form clumps. The mixture is centrifuged 

 until the corpuscles are thrown to the bottom of the tube and 

 the fluid above is clear, although it may be slightly straw 

 colored. The supernatent fluid is then drawn off with a 

 pipette. The corpuscles are washed free from serum and sodium 

 citrate by again filling the tube with normal salt solution, mix- 

 ing thoroughly and centrifuging until there again is a clear super- 

 natent fluid. If the material in the centrifuge tube is now ex- 

 amined, one sees a clear fluid above and a red fluid in the lower 

 part of the tube. The red fluid below, however, is not of the 

 same shade throughout. The uppermost part consists of a gray- 

 ish red layer, the leucocytes. These are layered above the red 

 blood cells because of the difference in specific gravity of the 

 leucocytes and red blood cells. It is from the gray layer that 

 the leucocytes, to be used in the determination of the opsonic 

 index, are obtained. 



To obtain the leucocytes the clear fluid above is drawn off 

 with a pipette and then with a clean pipette of about 1 . 6m. m. 

 inside diameter, the leucocytes are removed. To do this the 

 pipette firmly held in the hand, all of the air in the nipple 

 is expelled, and then as the pressure on the nipple is gradually 

 released, the open end of the pipette is held on the surface of the 

 uppermost layer of grayish red color. This fluid will contain 

 many red blood cells and also a relatively large number of leuco- 

 cytes. After this fluid has been drawn into the larger part of 

 the pipette the capillary end is sealed off. This fluid is called 

 "leucocytic cream." Wright and Douglas state that in their 

 experience there is no variation of the ability to engulf bacteria 

 within the space of a few hours but that after three days the 

 phagocytic power decreases to one-half or one-third of what it 

 was when freshly drawn. 



f 

 BACTERIAL EMULSION. 



Inasmuch as Wright had early decided that opsonins are 

 specific, such species or varities of bacteria are used in the deter- 

 mination of the opsonic index as may be of importance in the 

 bacterial infection. In the selection of the particular culture to 

 be used, there are two sources either the different cultures iso- 

 lated from the lesion are used or else the same species and varieties 



