THE MECHANISM OF AGGLUTINATION. 143 



already shown that a specific serum with very marked agglutinating 

 properties for red blood cells may be obtained by injecting an 

 animal of another species with a given blood.* Any acceptable 

 explanation then should be applicable, not only to the agglutina- 

 tion of bacteria but also to the agglutination of blood corpuscles 

 and, as we shall see further on, to the agglutination of particles of 

 casein suspended in milk. 



With this remark we may consider the different hypotheses that 

 have been proposed. 



1. Gruber's hypothesis. Gruber thinks that the agglutinin 

 changes the bacterial substance essentially. According to his in- 

 terpretation it renders the membrane of the micro-organisms more 

 viscous and this viscous condition of the superficial part of the 

 bacteria causes them to stick together and explains their collection 

 in definite clumps. 



This conception explains well enough why bacteria that have 

 once been united remain together, but it does not in any way explain 

 how the organisms approach each other to form the clumps. In 

 explaining the fact it emphasizes exclusively the structure of the cells 

 affected by the agglutinin, without admitting that the phenomenon 

 may be explained even in part by simple physical laws. Since this 

 hypothesis depends entirely on the supposed existence of a change 

 in a cell membrane, namely, a swelling accompanied by the pro- 

 duction of an adhesive substance, it in no way explains the agglu- 

 tination of inorganic chemical particles. It rules out any relation 

 between the agglutination of bacteria and the possible clumping 

 of chemical precipitates in a fluid. 



2. Bordet's hypothesis. The conception which we gathered 

 from a study of this phenomenon in 1896 is essentially different. 

 It appeared to us that in the case of the agglutination of the cholera 

 vibrio by its specific serum we have to deal with a phenomenon 

 in which the bacteria play only a passive role and in which their 

 vitality is not concerned. It is evident that motility is not neces- 

 sary, since agglutination occurs not only in bacteria that have lost 

 their motility, but also in red blood cells which are inert. The 

 passive role of bacteria is still further evident when the agglutination 

 of dead micro-organisms is considered. Gruber's hypothesis, more- 



* See article p. 134. 



