SPECIFICITY IN FERTILIZATION 217 



patible pollen facilitates the formation of substances 

 from the cells of the style that stimulate growth. 



If we attempt to seek the explanation along the lines 

 of serum reactions, an obvious possibility that various 

 authors have considered, it must be realized at the out- 

 set that for the present only broad analogies can be 

 considered; if a reaction comparable to immune reac- 

 tions is concerned in fertilization it may belong to a 

 new category and not to any of the recognized cate- 

 gories, whether of lysins, agglutinins, or precipitins. 

 The question would be whether a point of view could 

 be formulated consistent with the specificities both of 

 fertilization and of immunology. The analogy would 

 naturally be with the antigen X antibody reactions. 

 The principle of the reaction may be illustrated thus: 

 When an animal receives repeated injections of the red 

 blood cells of another species the serum acquires the 

 power of dissolving the red blood cells of the other 

 species (haemolysis); the reaction is specific in the 

 sense both of the species used and also of the kind of 

 cell used. In this case the blood cells injected consti- 

 tute the antigen, and the dissolving substance formed 

 in the blood of the other animal the antibody. In this 

 case it is found too that another antibody, an agglu- 

 tinin, is also formed in the serum of the other species. 

 This is more thermostable than the lysin and may there- 

 fore be demonstrated by heating the immune serum to 

 56° C, which destroys the haemolytic property, but 

 leaves the agglutinating substance intact. 



It has also been found that isohaemolysins and 

 isoagglutinins may be devel()i)ed in certain cases by 

 injecting blood cells of another indi\'idual of the same 



