18 ESSENTIALS OF BACTERIOLOGY. 



ological division of labor. (2) Certain elements may be differ- 

 entiated for the purpose of reproduction. 



The Staphylococcus pyogenes, the anthrax bacillus, and the 

 spirillum of relapsing fever are typical forms of the lower bac- 

 teria, while the actinomyces, or ray-fungus, is the most impor- 

 tant pathogenic member of the higher bacteria. 



FIG. 1. 



Micrococcus. Spirillum. Bacillus. 



Structure. Bacteria are cells ; they appear as round or cylin- 

 drical of an average diameter or transverse section of 0.001 mm. 

 (=1 micromillimeter), written 1 p. The cell, as other plant- 

 cells, is composed of a membranous cell- wall and cell-contents ; 

 " cell-nuclei " can in some cases be seen by the use of special 

 stains. 



Cell- Wall. The cell- wall is composed either of plant cellu- 

 lose, or a form of albumin, since it is less permeable than cellu- 

 lose membrane. The membrane is firm, and can be brought 

 plainly into view by the action of iodin upon the cell-contents, 

 which contracts them. 



Cell-Contents. The contents of the cell consist mainly of 

 protoplasm, usually homogeneous, but in some varieties, finely 

 granular, or holding pigment, chlorophyll, fat-droplets, and sul- 

 phur in its structure. 



It is composed chiefly of mycoprotein. 



Gelatinous Membrane. The outer layer of the cell-membrane 

 can absorb water and become gelatinoid, forming either a little 

 envelope or capsule around the bacterium or preventing the 

 separation of the newly-branched germs, forming chains and 

 bunches, as atrepto- and staphylo-cocti. Long filaments are also 

 formed. 



Zooglcea. When this gelatinous membrane is very thick, irre- 



