230 



INFECTION AND IMMUNITY 



individual microorganisms become more and more scarce, finally 

 leaving the medium between clumps entirely clear. While the clumping 

 of a motile organism suggests that motility has something to do with the 

 coming together in clumps, it nevertheless has no relation whatever 

 to agglutination, motile and non-motile organisms alike being subject 

 to the reaction. 



Microscopically observed, in small test tubes or capillary tubes, ag- 

 glutination evidences itself by the formation of flake-like masses which 



FIG. 60. MACROSCOPIC AGGLUTINATION. Dilutions from 1 in 10 to 1 in 1,000. 

 The first tube contains a 1 : 20 control with the bacteria and normal serum. 

 Agglutination complete in the tubes marked 10, 20, 50, 100. 



settle into irregular heaps at the bottom, leaving the supernatant 

 fluid clear, in distinct contrast to the even flat sediment and the clouded 

 supernatant fluid of the control. Macroscopically, too, agglutination 

 is evidenced when bacteria are grown in broth to which immune serum 

 has been added. Instead of evenly clouding the broth, the micro- 

 organisms develop in clumps or chains. 



Another phenomenon probably produced by agglutinins is the so- 



