CLASSIFICATION Q 



If living under unsuitable conditions it becomes quite short, 

 and stains solidly. Again, bacilli that are accustomed to appear 

 as short elements may grow to long threads without dividing, 

 or swell into unrecognizable form. Branching is sometimes 

 seen in rods and spirals, a condition due in certain cases to 

 involution, in others naturally among the higher bacteria. 



To measure bacteria, we use the thousandth part of a milli- 

 meter, called the micromillimeter, or micron, as the unit. The 

 Greek letter ju is the symbol for this unit. A micron is about 

 M5>ooo of an inch, yet a bacterium i n long, and % fj. in width, 

 is very large in comparison to some things that scientists measure, 

 such as the thickness of oil films, soap bubbles, or light-wave 

 lengths, in which the unit is a micromicron, and is symbolized 

 by /*/*. The shortest light-wave lengths are about 400 /*/*> or 

 .4 ju, while chromatic threads in cells of bacteria are often 100 w 

 in width. Then again there are many things smaller than these 

 threads. The thinnest part of a bursting soap bubble is but 

 7 juju in thickness. There are certain infectious agents that are 

 submicroscopic; that is, invisible even by the aid of Siedentopf's 

 ultraviolet microscope, which shows objects smaller by half a 

 light- wave length (.2 AIJU). 



The structure of the bacterial cell is very simple, consisting of 

 a delicate poorly staining limiting membrane or wall enclosing 

 a mass of substance with strong affinity for basic dyes like 

 methylene blue. Just what part of the bacterial interior is 

 cytoplasm and what is nucleus is not definitely known. Some 

 observers believe that all that is stained is chromatin, or nuclear 

 matter diffusely distributed through the bacterial cell, while 

 others think that a delicate cytoplasm exists under the wall and 

 that it is overshadowed by relatively great proportional bulk 

 of the nucleus. 



In the course of the rod we often see metachromatic bodies, 

 called the Babes-Ernst granules, and unstained spaces called 

 vacuoles, both of which are common to many bacteria. They 



