THE STRUCTURE OF BACTERIA. 13 



the typical forms recur only under quite defi- 

 nite conditions and that they vary according 

 to the nutrient media in which the bacteria 

 are growing. This holds true, indeed, to such 

 an extent that the prevailing form of a given 

 species must be assigned to one group or an- 

 other, according as it has been taken from one 

 or another medium. In this way bacteria 

 have been shifted from one class to another, as 

 for example is the case with the " miracle 

 germ " which, in accord with its common 

 form, was originally placed among the spher- 

 ical bacteria, but has been removed under the 

 bacteriological regime to the group of rod bac- 

 teria ; in truth, it may be rightly placed in 

 either group. 



After division the single cells may remain in 

 more or less intimate union (Fig. 4). The in- 

 dividual members of this association may either 

 preserve an independent existence, so that the 

 association is simply a loose aggregation of 

 cells, or the union may be so close that the 

 group of cells appears to form an organic unit. 



A. If division takes place in a single plane, 

 chains and filaments of cells arise ; the members 

 of the chain may be sharply defined one from 

 another or may be without apparent boundary. 

 In the latter case long filaments are formed 

 which, from their possession of an outer 



