100 Till-: AMEinCAX XATUBALIST [Vol. LV 



luted sera of man, apes and monkeys, respectively, it re- 

 acts to all, though in a varying degree. When mixed with 

 more highly diluted sera from such animals, it forms a 

 precipitate only with the serum of man and the manlike 

 apes (chimpanzee, orang-outang, gorilla), the chimpanzee 

 standing nearest to man. Absolute specificity may be 

 obtained if the antigen is sufficiently diluted. On the 

 basis of extensive experience, Uhlenhuth sets a dilution 

 of antigen of 1 to 1,000 as a standard beyond which no 

 precipitation will occur except with the specific antigen 

 employed in the sensitization. 



Thus the precipitin test became useful to the zoologist 

 in discriminating between different species, and it may 

 prove of importance in establishing the taxonomic posi- 

 tion of new forms, or in confirming or changing the classi- 

 fication of groups already known. The delicacy of the 

 test is remarkable. A properly sensitized serum may 

 give a reaction wdth blood diluted 20,000 or even 50,000 

 times. Sera have been obtained, indeed, in which specific 

 antigen could be detected in a dilution of 100,000. "When 

 one recalls that ordinary chemical tests cease to give 

 detectable reactions in blood diluted 1,000 times, he can 

 appreciate the value of these physiological methods of 

 measurement to the biologist. They apprise him of 

 species ditTeronces between the proteins of various ani- 

 mals which can not be determined by any known chem- 

 ical methods. 



The value of the precipitin test in forensic medicine, 

 in determining the nature of blood stains on clothing, 

 weapons or other objects, is well kno-sm to all of you, as is 

 doubtless their utilization in meat inspection, such as for 

 the detection of horse-flesh or dog-flesh in sausages or 

 other chopped meats, and in various other types of 

 adulteration. 



One tiling that interests the biologists greatly in the 

 precipitin reactions is the fact of so-called ''species spe- 

 cificity"— the fact that blood sensitized against one tis- 

 sue of a given foreign species w^ill react with extracts of 



