146 STUDIES IN IMMUNITY. 



surface of young and healthy bacteria. This superficial layer 

 includes the substance that is susceptible to attack and precipi- 

 tation by the agglutinin; when the agglutinin acts, this external 

 layer of the bacteria "swells up, becomes apparent and sticks to 

 the external layer of adjacent bacteria. Our opinion as to the 

 intimate nature of the phenomenon of agglutination is quite similar, 

 then, to that offered by Gruber and later defended by Roger. We 

 believe that agglutination consists in the coagulation and coales- 

 cence of the external layers of the agglutinable bacteria by the 

 agglutinating serum. " * 



As may be seen, Nicolle attaches or adds on Kraus' experiment 

 to Gruber's theory, but it is precisely this addition, on which the 

 whole value of the idea depends, that appears to us incomprehen- 

 sible and the weak point in the reasoning. Why should a precipi- 

 tation of the agglutinable substance within the external layer of 

 the bacterium, which we will not deny off hand, lead to a swelling 

 and viscosity which may bring about the coalescence and junction 

 of the external layers of neighboring organisms? 



However this may be, this interpretation, as well as Gruber's, 

 takes no account of the intervention of the physical laws of molec- 

 ular adhesion in explaining the fact. Nor does it presuppose 

 any relation between the collection of certain chemical precipi- 

 tates and bacterial agglutination, since its explanation of the phe- 

 nomenon depends essentially on the presence of a membrane and 

 external layer or of a ciliated covering susceptible to swelling and 

 stickiness. 



Nicolle describes a rather curious experiment in his article. He 

 found that the precipitate caused by the interaction of an active 

 serum with a culture filtrate has the property of carrying down with 

 it, in its clumping, inert particles like talcum powder in the form of 

 definite masses. Although this experiment is interesting we do 

 not think it is of any direct importance in explaining the phenome- 

 non of agglutination, its resemblance to which is only apparent. 

 These particles of talcum that collect into masses arc drawn together 

 mechanically and collected by a precipitate which is forming. To 

 admit that this non-specific phenomenon, which resembles super- 

 ficially true agglutination, is of importance, would be to admit 

 * Loc. cit., page 191. 



