Structure and Form. 3 



A ball, a short rod, and a corkscrew serve as convenient 

 models to illustrate these different forms. 



In size, the bacteria as a class are the smallest organ- 

 isms that are known to exist. Relatively there is consid- 

 erable difference in size between the different species, yet 

 in absolute amount this is so slight as to require the 

 highest powers of the microscope to detect it. As an 

 average diameter, one-thirty-thousandth of an inch may 

 be taken. If a hundred individual germs could be placed 

 side by side, their total thickness would not equal that of 

 a single sheet of paper upon which this page is printed. 



4. Manner of growth. As the cell increases in size 

 as a result of growth, it usually elongates in one direc- 

 tion, and finally a new cell- wall is formed, dividing the 

 -so- called mother- cell into two equal- sized daughter- cells. 

 This process of cell division, which is called, fission, is 

 continued almost indefinitely until growth ceases as a nat- 

 ural sequence. 



5. Cell arrangement. If this process of fission goes 

 on in the same plane it results in the formation of a cell- 

 row. A species forming such a chain of cells of the 

 coccus type is called strepto - coccus (chain- coccus). If 

 the second division plane is formed at right angles to the 

 first, a cell -surface is formed. If growth takes place in 

 three dimensions of space, a cell -mass is produced as in 

 the sarcina group. In some cases these cell aggregates 

 cohere so tenaciously that this character is of value in 

 distinguishing different species. 



6. Reproduction. The process of cell division known 

 as fission enables the bacterial form to reproduce itself 

 rapidly. ' Some species possess another method of vege- 

 tative reproduction, viz., spore-formation (fig. 1, g) . 

 The spores are usually formed within a mother-cell ; hence, 



