508 SPONTANEOUS AGGREGATION OF FLAGELLATES 



the life history of Bodo sulcatus. To settle this, the flagellates were 

 fixed after they had formed a band by introducing the fixative beneath 

 the cover-slip through a capillary tube previously placed in position. 

 Fixed in this way many individuals remained in position attached to 

 the glass so that the cover-slip could be lifted off the slide and then 

 the upper and lower flagellates stained and mounted separately. 

 The flagellates attached to the cover-glass were found to be identical 

 with those on the slide. We are thus dealing with a single species 

 but the possibility remains that the positively and negatively geo- 

 tropic individuals are in different stages of their life cycle. 



3. A further point that has not yet been mentioned is the following. 

 When an oxygen absorber such as reduced indigocarmine is introduced 

 beneath a cover-slip next to a suspension of Bodo, a band of flagellates 

 collects in a zone bordering the oxygen absorber in the manner already 

 described. After this principal band has formed a subsidiary band 

 containing relatively very few individuals makes its appearance nearer 

 in to the oxygen absorber than the principal band. The flagellates 

 in this inner band are close up beneath the cover-glass and move very 

 sluggishly in striking contrast to the intense activity of those in the 

 principal band. When the latter moves away to take up its definitive 

 position just inside the edges of the cover-slip, the inner band remains 

 for some time in place and then gradually the individuals composing 

 it dissipate to join the principal band. 



The reason for this inner band may be that a few individuals have 

 a lower oxygen optimum than the rest. This explanation seems 

 unlikely, however, because the inner band is not a permanent one; 

 after a certain time it dissipates. More probably some individuals 

 chance to swim into the region of lowest oxygen concentration and 

 are trapped there because, as was shown on page 504, the effect of 

 absence of oxygen is to inhibit swimming activity. This suggestion 

 is supported by the fact that in ordinary spontaneous band formation 

 an inner band is best developed in those preparations in which the 

 central region is cleared most rapidly. Here more individuals would 

 be left behind, trapped in the region of least oxygen. 



4. There are of course many kinds of bacteria present in the Bodo 

 cultures. In the preparations on slides many of the bacteria arrange 

 themselves in zones of optimum oxygen concentration in the manner 



