§ 3] CHEMOTAXIS 35 



peripheral zone — is apparent (Fig. 4). The peripheral zone 

 contains bacteria which are beyond the tactic action of the 

 oxygen. The central mass of bacteria have emigrated from 

 what is now a clear ring between the centre and the peripheral 

 zone. If the light be temporarily cut off, the central bacteria 



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Figs. 3-5 a. — Bacteria surrounding an algal cell. Fig. 2 shows the uniform distribu- 

 tion of the bacteria when the drop of water is kept in the dark. Fig. 3 shows the 

 aggregation of the bacteria towards the algal cell when this has emitted oxygen 

 rapidly in the strong light for two minutes. Fig. 4 shows the same preparation 

 shortly after the light has been cut off. Fig. 5 shows the same preparation when 

 a fainter light is now permitted to fall upon the green cell. Magnified about 170 

 diameters. (From Engelmann, '94.) 



begin to disperse (Fig- 5). If, a minute after, a dim light be 

 let through, the radius of its activity will be relatively small, so 

 that a central aggregation will be found and also an inner 

 peripheral zone, comprising those dispersing central bacteria 

 which are not affected by the small oxygen tension resulting 

 from the dim light (Fig. 5 a). 



