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which I can also fully confirm, that on these media which 

 already produce a rich growth of Pfeiffer's bacillus without 

 the assistance of foreign bacteria, the presence of other species 

 of bacteria does not further increase the growth. We might 

 at first imagine that in pure culture it would not be possible 

 to obtain such a good growth as when the growth-promoting 

 power of other bacteria was in action. That in actual fact 

 this is certainly not the case indicates that the „V" substance 

 in the haemoglobin derivatives and that formed by foreign bac- 

 teria, act upon Pfeiffer's bacillus in exactly the same manner, 

 and thus far they are to be considered identical. If they 

 had acted differently we should have expected that the pre- 

 sence of one of the substances by itself could not have produced 

 the same effect as the combined action of both. 



We must therefore employ „haemoglobin" in so low a 

 concentration that a pure culture is far from attaining its 

 maximal growth. But this is not all. It is further required 

 that the „haemoglobin" shall contain „X" in considerable ex- 

 cess of „V". In my experience an ordinary (impure) solution 

 of oxyhaemoglobin made by haemolysing horse red corpuscles 

 with distilled or tap water, has always satisfied this require- 

 ment, so that the same amount of haemoglobin which in the 

 absence of other bacteria can only support a weak growth 

 of Pfeiffer's bacillus and therefore only contains quite a small 

 amount of „V", has at the same time a sufficient quantity of 

 „X" to produce a maximal growth (that is to say, just as 

 good as in a pure culture on the best medium) when the 

 deficit of „V" is supplied by the colonies of a suitable spe- 

 cies of bacterium. 



That every „haemoglobin" solution does not lend itself to 

 the symbiosis test is shown by an experiment where the ., sym- 

 biosis" test consisted of inoculations from a culture of Pfeif- 

 fer's bacillus, on to agar plates containing haemolysed blood 

 corpuscles and others containing pepsin-digested blood. Both 

 media were prepared with several different concentrations 

 of „haemoglobin", so that in pure culture various degrees of 

 a weak growth of Pfeiffer's bacillus down to complete absence 

 of growth on the plates with the weakest concentrations, oc- 

 curred. On all the plates containing haemolysed blood the sym- 

 biosis reaction occurred in quite a typical form, while the 



