REPRODUCTION 89 



These moveinents may be observed quite readily, but only under the 

 correct conditions. If bacteria are examined in fluid culture or upon the 

 surface of an agar plate, nothing of the kind will be seen. But if the same 

 bacteria are examined either when set in the thickness of solid medium, or 

 growing between a block of medium and a coverslip upon its surface, then 

 the post-fission movements will become obvious. It was by the employment 

 of these techniques that they were first observed, and wliile it is entirely 

 accurate to describe bacteria as behaving in this manner, under these conditions, 

 the assumption that they do so under normal cultural conditions is quite 

 unjustifiable. The post-fission movements are merely the result of the growth 

 of the bacteria under conditions of severe mechanical restraint. Smooth 

 bacteria grow and elongate against the pressure of the surrounding medium. 

 When separation is complete, the daughter cells may be forced back again, 

 side by side, by the elasticity of the agar. Rough bacilU remain attached at 

 the point of division, and the filament, constrained in the same manner, is 

 compressed concertina-fashion, bending at the points of division. The division 

 of corynebacteria is more complex and will be discussed in Section E of this 

 chapter. It is sufficient here to say that the centre of the bacillus may be 

 very flexible, by reason of its multicellular structure, and the two halves may 

 remain attached even when forced into an acute angle. 



The mechanical constraint which brings about these appearances, by its 

 opposition to the growth of the bacteria, occurs to a very limited degree in 

 growth upon the surface of solid medium, and causes those pale shadows of 

 post-fission movements which contribute to the architecture of bacterial 

 colonies (Chapter VIII). 



D; COMPLEX REPRODUCTIVE METHODS 



(4, 5, 6, 7, 15, 18, 19) 



The most striking of the complex reproductive processes which accompany 

 and supersede simple fission in the later stages of a bacterial culture, is the 

 'filamentous growth and fragmentation, to which reference has already been 



