PROPERTIES OF COMPLEMENTS 45 



rabbit's serum or because the zymotoxic group of the rabbit's 

 complement is less active than that of the guinea-pig's com- 

 plement. In the former case a haemolytic dose of rabbit's 

 complement will prevent the combination of a hsemolytic 

 dose of guinea-pig's complement ; in the latter it will prevent 

 the combination of more. Our experiments are not suffi- 

 ciently extensive to give a definite statement on this point, 

 especially in view of the fact that during the progress of an 

 experiment the haemolytic action of complement may 

 diminish, and this change in the value does not always occur 

 in the two complements in the same proportion. We may 

 say, however, that we have obtained in several instances a 

 correspondence between the combining and the hsemolytic 

 ratio, i. e. a hsemolytic dose of guinea-pig's complement keeps 

 out a dose of rabbit's, and vice versa. We are inclined to 

 think that this may be the rule in the case before us, and 

 that probably the divergences in these ratios which we have 

 also met with are the result of accidental disturbing causes. 

 We do not consider this point as satisfactorily settled. It is 

 to be noted that the M.H.D. of the IB is practically the 

 same with the two complements used, provided that the 

 natural IB of the guinea-pig's serum for the ox's corpuscles 

 be first removed. The marked difference in the dosage of 

 the two complements would therefore depend upon there 

 being more complement molecules in the guinea-pig's serum 

 active towards the ox's corpuscles than in the rabbit's serum, 

 rather than upon differences in toxicity of the two comple- 

 ments. In other cases described below this does not hold ; 

 on the contrary, a great difference in the relative toxicity 

 of the complements of different animals is brought out 

 (p. 85). 



