66 M. F. GUYER AND E. A. SMITH 



rabbits. If the serum containing this substance is mixed with 

 horse-serum in vitro, a cloudiness results, due to the precipita- 

 tion of part of the protein in the horse-serum. The newly en- 

 gendered ingredient -of the sensitized rabbit serum which pre- 

 cipitates the horse-protein is termed a precipitin. The material 

 injected, in this case horse-serum, is spoken of as the antiqen. 

 Not only can precipitins be formed against the blood-serum of 

 an alien species, but against a wide range of substances, such 

 as bacterial products, milk, peptone, globulins, and various 

 albumins. 



It is this specificity against a distinctly alien albumin that 

 renders the precipitin test one of ready application in the medico- 

 legal differentiation of human blood and various human albu- 

 minous substances from those of other animals. 



Not only is the precipitin test useful in discriminating be- 

 tween non-related species, but it may prove to be important in 

 establishing the taxonomic position of new forms and in confirm- 

 ing or changing the classification of groups already known, since 

 among closely related species the specificity of the reaction is 

 not absolute. For instance, Nuttall, who made observation on 

 some 500 different animals found that rabbit-serum highly sen- 

 sitized with human blood-serum reacts, though in varying degree, 

 with the blood of all mammalia; a less strong serum, besides 

 reacting on human blood, also causes a precipitate in the blood 

 of anthropoid apes (chimpanzee, orang-outang, gorilla) and in 

 a less degree in the blood of other monkeys; whereas a weak 

 serum reacts with human blood and produces only a slight cloud- 

 iness in the blood of the anthropoid apes. He found that the 

 same quantitative differences exist in antisera specific for each 

 of the large vertebrate classes, birds, reptiles, anjl amphibia. 

 Thus by the precipitin test a differential scale of actual relation- 

 ships can be established. 



The degree of activity of sensitized sera is judged by the 

 dilution in which they will react. A properly sensitized serum 

 will give a distinct reaction in blood diluted 1000 times. . There 

 are records of reactions with blood diluted 20,000 times or even 

 50,000 times, while Ascoli, indeed, reports obtaining a specific 



