28 BACTERIOLOGY. 



The presence of a cell-wall is also demonstrated by the 

 action of iodine on the cells. An organism treated with 

 iodine solution and then fixed and stained will show the cell 

 membrane more or less distinctly. The protoplasm has 

 contracted from the wall and a number of colorless globules 

 or vacuoles will be seen, due to the cell fluid that has been 

 squeezed out. Moreover one or more heavily stained 

 roundish bodies, or "chromatin granules," can also be 

 observed. 



There is abundant evidence, therefore, which shows 

 that a definite membrane is present in all the stages of 

 the development of the bacterial cell. Moreover, plasmo- 

 lytic experiments indicate that the protoplasm is not firmly 

 united to this wall. The fact that strong solutions of salts 

 do penetrate the interior of the cell indicates that the cell- 

 wall is permeable. This of course is necessarily so, since 

 the nourishing material present in the surrounding fluid 

 must pass through this wall in order to reach the proto- 

 plasm. Similarly the waste products of the cell must pass 

 outward through a permeable cell- wall. 



The bacterial membrane or cell-wall is usually very 

 thin and colorless, and for this reason cannot be ordinarily 

 seen. As indicated above, it probably consists of a protein 

 and not of cellulose. Under special conditions the outer 

 layer of the cell-wall takes up water and gelatinizes. In 

 this case the cell becomes surrounded by a broad, colorless 

 zone which does not 'stain. This softened, expanded cell- 

 wall is known as the capsule. Owing to its soft, slimy 

 character it causes the cells to stick together, thus giving 

 rise to masses of cells which are, as it were, cemented 

 together. Such a mass, when touched with a wire, can be 

 drawn out into long, slimy thteads. This massed condition 

 of certain bacteria is designated as a zooglea. 



Well marked capsule formation is met with in only 

 comparatively few bacteria. In the majority of these 

 organisms it is either very feebly developed or entirely 



