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growth of Pfeiffer's bacillus made on the plates made with 

 pepsin-digested blood was only very slightly increased around 

 the coccus colonies. 



In another experiment the utility of the haemolysed blood 

 of the sheep, goat, rabbit, and pigeon was compared. The 

 haemoglobin of human blood has also been used in investi- 

 gating some of the strains of Pfeiffer's bacillus. The blood 

 of the first three species of animal and the human blood 

 gave quite as good „syml)iosis reactions" as horse blood, but 

 pigeon blood, — in several different concentrations — proved 

 to be quite unsuitable. 



We may imagine that the pepsin-digested (horse or sheep) 

 blood, and the haemolysed pigeon blood contain relatively much 

 „V", and thus have not the considerable excess of .,X" which 

 is necessary for a marked symbiosis reaction. Without the pre- 

 sence of foreign bacteria pigeon blood gave a considerably 

 richer growth than horse blood in the same concentration. 

 It was necessary therefore to reduce the concentration 

 of pigeon blood very greatly to get even a weak growth in 

 pure culture and so it is possible that there is too little „X" 

 to support a vigorous growth even with the addition of „V" 

 in large quantities. 



In the preparation of the haemoglobin solution, blood cor- 

 pusles have usually been used which were only partially freed 

 from 1 serum: To find out whether the amount of serum, — at 

 most, very slight, — which the medium contained, had any 

 effect on the „symbiosis", I arranged a simple experiment with 

 haemoglobin solution made from horse blood corpuscles which 

 were separated by the centrifuge and then washed three 

 times with salt solution. The symbiosis phenomenon on agar 

 with the serum-free haemoglobin occurred in just as marked 

 a degree as usual. (Davis (6) had the same experience). 



The next important point is that the growth-promoting or- 

 ganism must be inoculated at a distance from the culture of 

 Pfeiffer's bacillus, so that the two species of bacteria do not 

 come into contact with one another. The question of the 

 choice of the growth -promoting organism will be returned to 

 later. For the present I need only remark that for the first 

 few months I tried several different cocci and finally settled 

 upon a „white air-coccus", that is to say a Gram-positive mi- 



