THE FUNGI 241 
Keep watch of the cultures, looking at them daily or oftener. As 
soon as any change is noticed on the surface of a culture, make a 
descriptive note of it and continue to record the changes which are 
seen. Note the color of the areas of growth, their size, outline, ele- 
vation above the surface, and any indications of wateriness. Any 
growth showing peculiar colors or other character of special interest 
may be inoculated into freshly prepared culture media, using any 
additional precautions that are practicable to guard against con- 
tamination. 
309. Microscopic Examination. — Examine with h.p. some of the 
cultures. Place a particle of the growth on a slide, dilute it with a 
drop of clear water, and place a cover-glass over it. Note the forms 
and movements, also the sizes if practicable, of any bacteria that are 
found. Examine also scrapings from the surface-of the teeth and 
look for various forms of bacteria. 
310. Minute Structure. — Bacteria are spherical, rod- 
like, or spiral cells (Fig. 169). The cell-wall is thin and 
the protoplasmic contents usually colorless and apparently 
destitute of a nucleus. They are the smallest and among 
the simplest of known organisms. Extremely delicate 
“cilia are often found, either at one extremity of the cell 
or distributed over its general surface. The movements 
of bacteria are due to the lashing motion of the cilia. 
311. Life Habits of Bacteria. — As already stated (Sect. 
306), bacteria are either saprophytic or parasitic. Species 
of the latter group can often be artificially induced to live 
as saprophytes. 
Some kinds can only exist in the presence of free oxygen, 
while others cannot live in its presence. Those which 
require oxygen are more common and they flourish upon 
the surface of decaying animal and vegetable substances, 
causing them to putrefy. If meat, milk, eggs, or other per- 
ishable substances are sterilized by heating and then placed 
in sterilized vessels so arranged as to prevent any contact of 
