542 BIOLOGY OF PNEUMOCOCCUS 



place about ten days after the last injection of antigen. Barnes 

 and White 86 observed that the highest titers in rabbits were usu- 

 ally found in seventh-day bleedings, occasionally in those on the 

 twelfth day, and never when the blood was drawn on the twenty- 

 second day. The authors therefore concluded that the optimal 

 time for bleeding animals is in the period seven to ten days after 

 the last injection of a series. According to the schedule adopted, 

 the horses are bled on the ninth day after the last injection. 



Amount of blood. The amount of blood which may be withdrawn 

 from a horse depends on the size and physical condition of the ani- 

 mal. Usually from nine to twelve liters are taken. 



Yield of serum. The yield of serum is subject to variation and is 

 dependent in part upon the method used for collecting the blood, 

 the number of bleedings to which the horse has been subjected and, 

 apparently, to other more obscure factors. The yield obtained by 

 using weights as an aid in expressing serum from the clot, as de- 

 scribed by Avery and his associates, was from 50 to 55 per cent. If 

 suitable equipment is available, the blood may be drawn directly 

 into bottles of several liters capacity, and centrifuged after the 

 clot has formed. In this manner the percentage of serum obtained 

 may be slightly higher. When no special methods are employed and 

 serum expressed by simple coagulation is drawn, the yield is likely 

 to average only about 30 to 35 per cent of the total volume of 

 blood. 



There has been an impression in some laboratories that the 

 amount of serum obtained from the first few bleedings was less 

 than that from later bleedings. An analysis of results from a small 

 group of horses under the observation of the authors of the pres- 

 ent volume supports this belief. The data are taken from records 

 of horses that had been bled from twenty-one to forty-six times. 

 The average percentage yield was calculated for the first three, the 

 middle three, and the last three bleedings, respectively, of each 

 horse. Approximately 8 per cent more serum was obtained from 

 the middle three bleedings as compared with the first series, and 



