PRECIPITINS 143 



leads to a disappearance of precipitins from the 

 blood. 



Collecting the Serum. When the animals have 

 received five to six injections, and some days have 

 elapsed it is well to draw off samples of the blood 

 and to test for precipitins. This is easily done by 

 shaving the ear and cleansing the skin with alcohol 

 and sterile water. An incision is then made into 

 the marginal vein and a few drops of blood collected 

 in a small test-tube. This is then set aside to allow 

 the blood to coagulate. After the serum has sepa- 

 rated it can be tested and if it prove insufficiently 

 powerful, treatment may be continued, otherwise 

 the animal may be killed, preferably a week or ten 

 days after the last injection. The animals may be 

 killed in a variety of ways. Uhlenhuth chloroforms 

 them, opens the thoracic cavity under aseptic 

 precautions, and, cutting through the beating heart, 

 the blood is allowed to flow into the thoracic cavity, 

 whence it is removed by means of sterile pipettes 

 to suitable vessels. Nuttall's method is to shave 

 the neck and disinfect the skin with lysol solution; 

 bend the animal's head backward to put the skin 

 of the neck on the stretch, and have an assistant 

 make a clean sweep with a sterilized knife through 

 the tense skin to and through the vessels. The 

 blood spurts into a large sterile dish which is 

 immediately covered when the main flow has ceased. 

 The dishes are placed horizontally until a clot has 



