114 



INTRODUCTION TO EVOLUTION 



SHEEP 

 SERUM 



SERIAL DILUTIONS 



RABBIT SERUM 



ANTISERUM CONTAINING 



ANTIBODIES AGAINST 



-\ SHEEP SERUM 



PLACED 



IN 



EACH 



TUBE 



GO A T SERUM _ 



SERIAL DILUTIONS 



M rip OP 



BEEF SERUM _ 



SERIAL DILUTIONS 



DOG SERUM 



SERIAL DILUTIONS 



_/ 



ry^f NO. 



7 8 9 10 



FIG. 6.2. Principle of the precipitin test, employing serial dilutions of antigen, and the 

 interfacial reaction. 



sheep serum is placed in the bottom of each of a series of test tubes. Each 

 tube then receives a small quantity of a dilution of sheep serum. The first 

 tube in the series receives sheep serum diluted 1 : 100 with physiological salt 

 solution. The second tube receives a 1 :200 dilution of the sheep serum. In 

 succeeding tubes sheep serum in dilutions of 1 :400, 1 :800, 1 : 1600, 1 :3200, 

 1 : 6400, 1 : 12,800, and so on, is added. The diluted serum is added carefully 

 so that it will not mix with the antiserum already present. Thus a sharp 

 interface is left between the two; at this interface a ring of precipitate 

 forms. Such a "ring test" is more delicate than the form of the test de- 

 scribed previously, in which the solutions are mixed and the precipitate 

 settles to the bottom of the tube. 



After the rings have had time to form, the highest dilution in which 

 ring formation occurs is determined by inspection. This dilution is called 



