2 GENERAL MORPHOLOGY AND BIOLOGY. 



groups will be more conveniently taken later (p. 16). In 

 some cases, especially among the bacilli, there may occur 

 under certain circumstances changes in the protoplasm 

 whereby a resting stage or spore is formed. 



The higher forms show advance on the lower along two 

 lines, (i) On the one hand they consist of filaments made 

 up of simple elements such as occur in the lower forms. 

 These filaments may be more or less septate, may be pro- 

 vided with a sheath, and may show branching either true or 

 false. The minute structure of the elements comprising 

 these filaments is analogous to that of the lower forms. 

 Their size, however, is often somewhat greater. The lower 

 forms sometimes occur in filaments, but here every member 

 of the filament is independent, while in the higher forms 

 there seems to be a certain interdependence among the 

 individual elements. For instance, growth may occur only 

 at one end of a filament, the other forming an attachment 

 to some fixed object. (2) The higher forms, moreover, 

 present this further development that in certain cases some 

 of the elements may be set apart for the reproduction of 

 new individuals. 



Terminology. The term bacterium of course in strict- 

 ness only refers to the rod-shaped varieties of the group, 

 but as it has given the name bacteriology to the science 

 which deals with the whole group, it is convenient to apply 

 it to all the members of the latter, and to reserve the term 

 bacillus for the rod-shaped varieties. Other general words, 

 such as germ, microbe, micro-organism, are often used as 

 synonymous with bacterium, though, strictly, they include 

 the smallest organisms of the animal kingdom. 



While no living organisms lower than the bacteria are 

 known, the upper limits of the group are difficult to define, 

 and it is further impossible in the present state of our 

 knowledge to give other than a provisional classification of 

 the forms which all recognise to be bacteria. The division 

 into lower and higher forms, however, is fairly well marked, 

 and we shall therefore refer to the former as the lower 

 bacteria, and to the latter as the higher bacteria. 



