CHAPTEE II. 

 SIZE AND STRUCTURE OF THE BACTERIAL CELL. 



Bacteria have been described as uni-cellular micro- 

 organisms. The fact that they consist of a single cell 

 indicates at once their microscopic character. The micro- 

 scope reveals a world of animate beings which differ 

 enormously in size. Some of these organisms may be large 

 while others are exceedingly small. The bacteria are 

 usually spoken of as the smallest of living beings, the 

 "infinitely small." Until very recently no form of life was 

 known smaller than these. The discovery of the microbe 

 of pleuro-pneumonia has revealed the existence of organ- 

 isms considerably smaller than the bacteria. This new 

 organism is so small that the very highest magnification of 

 the microscope still leaves its form uncertain. 



The size of microscopic objects is usually expressed 

 in micro-millimeters. A micro-millimeter or micron may 

 be defined as the thousandth part of a millimeter. Inas- 

 much as a millimeter corresponds approximately to it 

 of an inch, it follows that one micro-millimeter repre- 

 sents ism of an inch. It is customary to speak of a 

 micro-millimeter as a micron -and to designate it by the 

 Greek letter fi.. 



The largest micrococcus known has a diameter of 2 /t or 

 tnhni of an inch. There are some micrococci that have a 

 diameter only one-tenth as large as this. The common 

 pus-producing micrococci have a diameter of about 0.8 ^ or 

 iriiinr of an inch. Inasmuch as the diameter of a red blood 

 cell is about 8 //-, or about ^bW of an inch, it is evident that 

 ten of these cocci placed in a row would correspond to the 



