496 SPONTANEOUS AGGREGATION OF FLAGELLATES 



reduced by a certain amount throughout the slide that the Bodo feel 

 the attraction of the central region. They are thus only sensible to 

 this influence when in the presence of an oxygen concentration closely 

 approaching the optimum for them. 



This was confirmed by an experiment with an oxygen absorber 

 such as reduced indigocarmine or pyrogallic acid. Two slides were 

 prepared with long cover-slips supported by pieces of glass rod. 

 Under one cover-slip was run in some Bodo suspension taken straight 

 from the culture jar and under the other some of the same culture 

 which had been aerated by exposure in a petri dish. The liquid was 

 not allowed in either case completely to fill the place beneath the 

 cover-slip, but a space was left at one end into which the oxygen 

 absorber was to be run. When this had been done, it was seen that 

 the flagellates in the aerated preparation collected much later into 

 the region of reduced oxygen concentration next to the indigocarmine 

 than they did in the non-aerated preparation. In fact whereas in 

 the non-aerated liquid the Bodo moved at once into the region next 

 the oxygen absorber, in the aerated liquid they did not do this until 

 by their own respiration they had reduced the oxygen content of the 

 general suspension to a point approaching the optimum for them. 

 The flagellates thus gave no response to lowered oxygen concentration 

 when in the presence of an amount of oxygen much above their 

 optimum. 



It was mentioned at the commencement of this account that in a 

 normal cover-slip preparation the Bodo ring may become established 

 in 2 minutes or it may take 2 hours to form. A series of experiments 

 demonstrated that this great difference in time is due to different 

 initial oxygen contents of the suspensions. If the suspension to be 

 used is exposed to the air for some time during the preparatory 

 filtering operation it will have a higher oxygen content than liquid 

 taken straight from the culture jar, for the oxygen concentration in 

 the cultures is always considerably lower than the saturation con- 

 centration under atmospheric partial pressure, and therefore in the 

 filtering through bolting-silk oxygen is absorbed from the air. Aggre- 

 gation and ring formation always take place later in a suspension 

 that has been exposed to the air than in one that has not. 



