MORPHOLOGY AND RELATIONSHIPS OF MICROORGANISMS 31 



ing hog cholera, for example, is so small that it will pass through 

 the pores of a fine porcelain filter, and will cause disease when 

 injected into a healthy pig. Such an organism is frequently spoken 

 of as ultramicroscopic, or as a filterable virus. 



Histology and Structure of Bacteria. This topic may be treated 

 under four subheads, the cell wall with its related sheaths and cap- 

 sules, the protoplasm, the cell inclusions, and the flagella. 



Cell Wall. The bacterial cell is in all cases surrounded by a 

 definite membrane that morphologically resembles the cell wall 

 of higher plants. When tested chemically with various reagents 

 and examined microscopically, it is sometimes found to give 

 the reactions characteristic of chitin, the material which makes 

 up the hard outer shell of insects, and is found as a cell mem- 



Fig. 9. Capsulated bacteria. 



brane in many animals. Chemically chitin is an ammo-sub- 

 stitution product of a carbohydrate. The fact that the cell 

 walls in bacteria so frequently resemble in composition those 

 of certain animals has been used as an argument for the animal 

 relationships of the bacteria. This is negatived, however, by 

 the fact that in numerous molds and other fungi, undoubted 

 plants, the cell walls are made up of a similar substance. 



The cell wall in bacteria is usually covered by a layer of mucil- 

 aginous material, in most cases so thin that the most careful technic 

 must be employed in its demonstration, in other cases a thick 

 coating or capsule. The nature of this capsular substance has 

 been a fertile subject for dispute. A few bacteriologists have 

 claimed that it is composed of living protoplasm, the majority, 



