44 BACTERIOLOGY. 
and devoid of appendages; but the longer rods and 
spirals are usually provided with fine hair-like append- 
ages or flagella, which are their organs of motility. 
These flagella, either singly or in numbers, are some- 
times distributed over the entire body of the cell, or 
they may form a tuft at one end of the rod, or only 
one polar flagellum is found. The polar flagella 
appear on the bacteria shortly before division. The 
nature of flagella is little understood; they are believed 
by some to be formed of protoplasmic material which 
" penetrates the cell membrane, and probably have the 
property of protrusion and retraction. So far as we 
know, the flagella are the only means of locomotion 
possessed by the bacteria. They are not readily 
_ stained, special staining agents being required for 
this purpose. The envelope of the bacteria, which 
usually remains unstained with the ordinary dyes, 
then becomes colored and more distinctly visible than 
‘is commonly the case. Occasionally, however, some 
portion of the envelope remains unstained, when the 
flagella present the appearance of being detached from 
the body of the bacteria by a narrow zone. Unfor- 
tunately, many of the methods employed for staining 
flagella cause them to become degenerated, so that their 
perfect demonstration is often very difficult. In cul- 
tures of richly flagellated bacteria peculiar pleated 
masses sometimes are observed, consisting of flagella 
which have been detached and then matted together. 
Bacteria may lose their power of producing flagella for 
a series of generations. Whether their power be per- 
manently lost or not we do not know. 
The vegetative reproduction of bacteria takes place by 
division. When development is in progress a single 
