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 

 loot 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 



