96 EVOLUTION AND GENETICS 



bodies having similar actions to those artificially produced as a 

 result of immunization with this or that substance, we know of 

 normal agglutinins, haemolysins, bacteriolysins, antitoxins, anti- 

 ferments, etc., all of which go to prove the correctness of Ehrlich's 

 views in this respect." 



The theory of antibody formation and structure is complex in 

 its details. For our purpose it is sufficient to note that the various 

 types of antibodies are antitoxins, antiferments, cytotoxins of 

 various kinds, agglutinins and precipitins, each named for the 

 substance whose presence in the serum gives rise to it, or for its 

 action upon that substance. 



Precipitins. Thus the precipitins have the power to form a pre- 

 cipitate when mixed with the substance which has produced them. 

 Little is known of their nature, but the precipitates produced by 

 their reaction with proteins of blood sera show characteristics of 

 proteins in several recorded cases. 



Preparation of Precipitins. Nuttall's procedure involved the 

 use of rabbits, chiefly, as the source of antisera (i.e., sera contain- 

 ing antibodies). Injections of the blood or sterilized blood serum of 

 other animals were made at intervals of several days until three to 

 twenty had been administered. After the completion of this 

 treatment six to fifteen days were allowed to elapse, the rabbit 

 was then killed and bled, and the blood serum extracted and pre- 

 served with the necessary precautions to maintain its sterility. 

 In this way antisera were developed for the blood of a number of 

 species of animals, as well as for other substances, such as cows' 

 milk and egg albumen. 



Nuttall states that "we have sufficient evidence to show that 

 precipitins are not formed in the serum of closely related animals." 

 He cites the experiments of Bordet and Hamburger, in which 

 precipitins were not produced when rabbit serum was injected into 

 guinea-pigs. Nolf likewise found it impossible to produce antisera 

 in pigeons treated with fowl serum. This indicated a sufficient 

 similarity in the bloods of the animals concerned to account for 

 tolerance of the one species for the blood of the other, an evidence 

 of blood relationship which harmonizes with the fact that the 

 animals are related in a morphological way. 



Precipitin Tests. In using the antisera thus prepared, samples 

 of sera were collected in two ways, viz., fluid and dry, although 

 the latter method proved to be the more practicable. Dilutions 



