H. MUNRO FOX 507 



ary position. The relative density of flagellates in the upper and lower 

 parts of the band varies in different preparations, being sometimes 

 more and sometimes less marked. The degree of slope of the section 

 of the band also varies, being sometimes more and sometimes less 

 steep. The consequence of the greater concentration of Bodo at top 

 and bottom of the band and of the slope of the latter is that when 

 the preparation is viewed from above the band appears to be double 

 (Fig. 18). The outer and inner lines of this double band naturally 

 appear the further separated from one another the higher the cover- 

 glass is placed above the slide. 



••:!f.-y.-::-a:\r>!-i 



lilL -Cr 



Fig. 17. 



Fig. 18. Fig. 19. 



Fig. 17. Vertical section of a flagellate band, a, cover-glass; b, slide; c, 

 water-air surface. 



Fig. 18. Enlarged view of one corner of a square flagellate band, a, edge of 

 cover-glass. 



Fig. 19. Vertical section through an air bubble and a ring of flagellates sur- 

 rounding it. a, air bubble. 



The section of the band has a similar shape to that of the water- 

 air surface at the edge of the cover- slip. This is shown in Fig. 17. 

 It was thought at first that the form of the water-air surface might 

 determine the form of the band, but this is not the case for a flagellate 

 ring surrounding an air-bubble has the same sectional shape as the 

 main band whereas the section of the water-air surface bordering 

 the bubble is a symmetrical curve as shown in Fig. 19. The reason 

 for the peculiar shape of the Bodo band remains undetermined. It 

 is unknown why those individual flagellates which are negatively 

 geotropic seem to have a lower optimum oxygen concentration than 

 those which are positively geotropic. The possibility naturally arose 

 that there were two species of Bodo present or two different stages in 



