H. MUNRO FOX 



489 



atmosphere. In the center of the preparation no exchange with the 

 air can take place, so that here the concentration of dissolved oxygen 

 will decrease and that of carbon dioxide increase most rapidly. 



It seems probable, then, that the flagellates collect in the central 

 region because they are attracted either (a) into a region of higher 

 hydrogen ion concentration, or (b) into one where the concentration 

 of dissolved oxygen is lower. Which of these alternative explanations 

 is the correct one? It was attempted to answer this question by 

 isolating the two possible causes and allowing each to act separately. 



To test suggestion (a) a long cover-glass was supported over a 

 slide by wax feet placed beneath its four corners. Some filtered 

 suspension of Bodo was then let in under the cover-glass from one 

 end, so as not to fill completely the space between the cover-glass 



Fig. 9. Even distribution of flagellates beneath a long cover-glass unaffected 

 by the introduction of carbonic acid at one end. a, carbonic acid. 



and the slide. Immediately afterward from the other end of the 

 cover-glass some water which had been saturated with carbon dioxide 

 was let in (Fig. 9). If such a solution had an attractive influence 

 on Bodo, the flagellates would have collected in the region where the 

 two liquids merged. This they did not do. 



To test suggestion (b) a similar preparation was made but in 

 place of carbonic acid, reduced indigocarmine was introduced 

 beneath one end of the cover-slip. Indigocarmine (sodium sulfindi- 

 goate) was reduced in the absence of oxygen (in a stoppered bottle) 

 by a solution of 1 per cent glucose containing 1 per cent caustic 

 potash. By this means the yellow leuco base is formed. As much of 

 a concentrated solution of indigocarmine was used as the glucose 

 would turn from blue to yellow in 1 hour in the stoppered bottle. 

 In the presence of oxygen the yellow leuco base reoxidizes instantane- 



