H, Gr. Thornton 



251 



As the moisture appears, a slow motility can be observed in the 

 colony, the rods sliding over one another and slowly pushing outward 

 the edge of the colony. Single rods do not appear capable of overcoming 

 the surface tension at the edge of the water film. They have many times 

 been observed pushing outward but do not force their way out from the 

 colony^. Packets of six or more cells, however, are able slowly to press 

 outward the edge of the water film. Consequently, where a packet of 

 cells lies at the edge of the colony, in such a manner that the rods lie 

 radially, or at right angles to the film edge, they may often be observed 





Fig. 1. Bacillus deiidmides. Successive stages in the formation of a process 

 from a 6 hours old surface colony on nutrient agar. 



to force their way outward producing a small promontory (Fig. 1). But 

 where the outer packets lie so that the cells are oriented tangentially, 

 they do not press away from the centre. The out-pushing of the colony 

 edge is therefore discontinuous so that the colony becomes irregular or 

 lobate. As the water film becomes thicker, the cells move about more 

 actively, and, in the interior of the colony, they often lose their arrange- 

 ment in packets, and where there is most moisture a streaming move- 

 ment of cells may be observed. In each projection of the colony edge, 

 the cells tend to swim outward and to collect at the distal extremity, 



1 An attempt was made to modify this condition by adding 0-05 per cent, saponin to 

 the medium in order to lower the surface tension of the film. The saponin, however, 

 caused abnormal growth of the organism. 



