128 THE PRINCIPLES OF IMMUNOLOGY 



tion cannot be done immediately the clot may be allowed to separate in the 

 ice chest. As soon as the serum has separated out of the clot, the tube is 

 centrifuged and the serum collected by means of a pipette. If guinea-pigs are 

 large, the blood may be collected in smaller quantities by heart puncture or by 

 bleeding from an ear vein, thus obviating the necessity for killing the animal. 



PIG. 14. Method of obtaining blood from guinea-pig (See text). 



Origin of Complement. Considerable controversy has been waged 

 concerning the origin of complement since the time that Hankin and 



