12 MORPHOLOGY 
plast is invested by a membrane, appears to be homogeneous, and usu- 
ally contains a few granules. It seems not to have even such simple 
differentiation as occurs in the cells of certain Cyanophyceae; but in 
all such cases it must be kept in mind that we are dealing with very 
minute objects and that presently a technique may be developed that 
will reveal an amount of organization that we have no means of seeing 
at present. 
Many of the bacteria are ciliated, the cilia being distributed over the 
body in various ways, and always extremely difficult to detect. These 
ciliated forms are very active, and their movements have suggested 
that bacteria are animals. Under certain conditions many bacteria 
pass into a quiescent stage and collect in colonies that are held together 
by a gelatinous matrix formed from the walls. These quiescent colonies, 
thus embedded, form characteristic pellicles on nutrient media, as on a 
decoction of hay, on bouillon, on stagnant water, and on various solid 
media. This quiescent, pellicle-forming stage is known as the zoogloea 
stage. 
Multiplication. The multiplication of cells by division is exceedingly 
rapid, the progeny of one cell in twenty-four hours often running into 
many millions. As already said, these newly formed cells either sepa- 
rate or hang together in filaments. When the nutritive supply fails, 
the protoplasm condenses in the middle or end of the cell and becomes 
invested by a heavy membrane. These are the so-called spores, but 
they are really resting cells such as are formed among the Cyanophyceae, 
except that they are formed within the old cell. These resting cells are 
even more resistant than the ordinary vegetative cells. In favorable 
conditions the protecting membrane bursts and the protoplast resumes 
active division. 
Cultures. It is very difficult and often impossible to recognize species 
of bacteria by the appearance of the individual cells, but in mass cultures 
the colonies are often very distinct in form, color, structure, and effect 
on nutrient media. These mass cultures are made in liquid media or 
upon solid media (gelatin, agar, potato, etc.). For purposes of inves- 
tigation pure cultures are absolutely necessary, which means the sepa- 
ration of the form under investigation from every other form with which 
it may be associated, a process requiring a special technique. 
Activities. Many bacteria are peculiar in that they are able to live 
in the absence of free oxygen, which in other plants is associated 
with the fundamental process of respiration. Such bacteria are called 
