I9I4] J- Bronfenhrenner, W. T. Mitchell, M. J. Schlesinger 3^9 



any human or animal fresh serum, and would permit the examina- 

 tion of serum taken at any time, no matter what the condition of 

 the patient might be. This modified procedure is the following: 

 After remaining over night in contact with the suspected serum in 

 the ice-box, the placenta (or other substratum, as the case may be) 

 is centrifuged, washed with water to remove any serum that may 

 stick to it, and placed in a new thimble with any serum that hap- 

 pens to be on hand. The best for this purpose is serum from a 

 guinea-pig kept without food long enough (6-8 hours) to free its 

 blood from dialysable reactive substances. It is necessary, of course, 

 to take the additional precaution, in the diagnosis of pregnancy, of 

 using male guinea-pig serum, since fresh serum from a pregnant 

 guinea-pig gives a positive reaction with human placenta, and may 

 thus cause grave error. 



