H. MUNRO FOX 509 



originally described by Beyerinck.^ The bacteria, however, do not 

 commence by forming obvious central aggregations which then become 

 converted into rings. This is, perhaps, because they are less motile 

 than the iiagellates : they collect more slowly than the latter into their 

 zones of optimum oxygen. 



Now when a bacterial optimum position coincides with that of Bodo, 

 there is frequently an antagonism between the two organisms, the 

 bacteria preventing the flagellates from taking up their normal posi- 

 tion. In particular, there were frequently present certain cocci the 

 optimum position for which was situated at the middle depth of the 

 Bodo band and certain bacilli whose optimum coincided with the lower 

 and outer portion of the Bodo band. When a preparation was made 

 from a culture in which these bacteria were present in quantity the 

 Bodo were excluded from the middle and lower parts of their zone and 

 all forced to occupy the upper and inner part. The bacteria took up 

 their position more slowly than the flagellates, consequently when 

 the band first formed the flagellates occupied both upper and lower 

 regions, as shown in Fig. 17, but when the bacteria arrived they drove 

 the flagellates out of the lower region. When a preparation having 

 a Bodo band with these bacteria in its lower part and a piece of moss 

 cutting across the band as described on page 498 is removed from 

 darkness to light, all the flagellates and bacteria move inwards to 

 take up a new position, as shown in Figs. 11, 12, and 13 (Part I). 

 But the Bodo move back much more rapidly than the bacteria and, 

 arriving first at the new position of optimum oxygen, are able to 

 place themselves in a band extending in depth from cover-glass to 

 slide. Later on the bacteria arrive and drive out the flagellates 

 from the lower and middle parts of their zone. 



SUMMARY. 



Spontaneous aggregations of flagellates are formed under the cover- 

 glass because the organisms are attracted to and remain in regions 

 where the concentration of dissolved oxygen is less than the saturation 

 concentration under atmospheric partial pressure. These regions of 



^ Beyerinck, M, W., t)ber Atmungsfiguren beweglicher Bakterien, Centr. Bakt., 

 1893, xiv, 827. 



