BACTERIA. 39 



and maimed in society and are to be accounted for by the fact that they 

 are deformed by their own by-products. In fact, it is quite probable that 

 they are autogenic. Involution forms are very likely to occur in artificial 

 culture and are much more common with some species than with others. 



SIZE. 



The bacteria were formerly spoken of as the smallest of living things, but 

 since the recognition of the ultramicroscopic organisms it is necessary 

 to be somewhat more specific in characterizing their dimensions. The 

 unit of measurement in microscopy is the micron (/*), or micromillimeter. 

 This is .001 of a mm. or approximately 1/25000 of an inch. Applying 

 this unit to the bacteria we find that the micrococci and the short diameter 

 of the bacilli and spirilla average about i/z. The micrococci vary 

 in diameter from a small fraction of a micron to three or four microns 

 in diameter. The bacilli are sometimes very small, as the influenza 

 bacterium with a width of 0.2/1 and a length of o.$/j., and sometimes very 

 large as, for example, the Bact. anthracis with a width of i.2/ and a 

 length of 5.20 JJL. The spirilla average about i.o/z in diameter but may be 

 as long as 30^-40^. 



MOTILITY. 



When bacteria are viewed under the microscope in a living condition 

 many of them are seen to move. This movement may be one of two 

 kinds. In some cases it is progressive, the individuals move about from 

 one part of the field of the microscope to another and change their relative 

 positions. In other cases the movement is vibratory, they move back and 

 forth but do not progress or change their relative positions to any extent. 

 This latter form of movement is known as brownian movement, because 

 it was first described by Brown. 



BROWNIAN MOVEMENT. This movement is probably caused by the 

 impact of the molecules of the suspending medium and for this reason 

 is sometimes called molecular movement. It is not characteristic of 

 bacteria, or indeed of life, but is shared by all small microscopical 

 objects when suspended in a fluid medium. Most beautiful examples 

 of brownian movement can be se^n by suspending granules of India 

 ink or carmine and examining them under the microscope. This 



