26 BACTERIOLOGY. 



There is reason to believe that the cell-wall proper 

 consists chiefly of protein substances. The cell-wall may 

 be considered as a layer of hardened or condensed proto- 

 plasm. It is stained by anilin dyes and is not digested by 

 proteolytic ferments such as pepsin and trypsin. It appar- 

 ently responds to Millon's reagent, but is insoluble in 

 Schweitzer's reagent, which, as is known, dissolves cellu- 

 lose. It is quite possible that certain species, especially 

 when growing on special media, acquire some cellulose-like 

 substance. In other species dextrin-like products seem to 

 be elaborated. Again, granulose, or a chitinous substance 

 may be present at times in the cell-wall. 



The existence of a cell-wall is indicated by various 

 characteristics exhibited by bacteria. Thus, the constant 

 form of a spirillum or of rods can only be accounted for by 

 the presence of an outer hard envelope. Occasionally the 

 contents of a cell may die out and disappear, in which case 

 the empty shell remains and is easily distinguished from 

 the normal cells, which stain perfectly. 



The presence of a cell-wall is especially clearly demon- 

 strated in certain cases. Thus, the contents of the cell 

 contain, in addition to the protoplasm, more or less of a 

 watery cell fluid. As a result of surface tension this may 

 gather in minute droplets between the semi-solid proto- 

 plasm. Occasionally the protoplasm will show marked 

 adhesion to the wall, and as a result the fluid is prevented 

 from forming globules and causes a separation of the con- 

 tents of the cell into discs. A disc of protoplasm will 

 alternate with a disc of the cell fluid. On staining such an 

 organism it will exhibit transverse bands. 



The cell fluid holds in solution various mineral salts 

 which exert a certain osmotic or internal pressure which is 

 counteracted by the protoplasm and especially by the firm 

 outer cell-wall. The normal internal pressure of the cell 

 contents depends upon the composition of the fluid sur- 

 rounding the cell. When the cell is placed in water which 



