H. MUNRO FOX 495 



trolling factor in the behavior of the flagellates. Of these I shall 

 mention three. They are: (1) the effect of the original oxygen con- 

 tent of the Bodo suspension on the rate of band formation; (2) the 

 reduction of oxyhemoglobin by Bodo; and (3) the effect of a green 

 plant in sunlight on the band. 



To show the effect of the oxygen content of the suspension of flagel- 

 lates on the rate of aggregation and band formation two preparations 

 were made on slides in the manner already described. In the first 

 preparation the liquid was let in beneath the supported cover-glass 

 immediately after having been pipetted out of the culture jar. For 

 the second preparation a pipetteful of liquid was taken from the 

 same culture jar, placed in a petri dish, and exposed to the air for a 

 short time before being let in under the cover-glass. A Bodo culture 

 always contains less oxygen in solution than plain water in a similar 

 jar at the same temperature. This is of course due to the fact that 

 the flagellates are continually absorbing oxygen in respiration. Con- 

 sequently the suspension of Bodo which had been exposed to the air 

 in the petri dish had acquired a greater oxygen content than that 

 taken directly from the culture jar. It was found that the aggrega- 

 tion of flagellates was much slower in the aerated than in the non-^ 

 aerated preparation and that the central clear area became established' 

 later in the former. 



It was to be expected that the central clear area bordered by the' 

 band of flagellates would become established later in the aerated prepa- 

 ration. Here it must take longer for the organisms to consume suffi- 

 cient oxygen to make the central region untenable for them. But 

 it is not at first obvious why the central aggregation should form 

 later in the aerated preparation. It would be expected, rather, that 

 when in the presence of more oxygen than the optimum the flagellates 

 would migrate from any region of higher into any region of lower 

 oxygen concentration. Now a region of relatively lower oxygen con- 

 centration due to the respiration of the flagellates must arise as soon 

 in the center of the aerated as of the non-aerated preparation; never- 

 theless, in the former the organisms are not attracted so soon towards 

 the center as they are in the latter. It seems thus that when the 

 liquid is well aerated the flagellates are not sensible to a region of lower 

 oxygen concentration: it is not until the oxygen content has been 



