26 THE MORPHOLOGY OF BACTERIA 



rigid and maintains the shape of the organism. Generally speaking, 

 this cell membrane is intermediate in character between that char- 

 acteristic of animal and of plant cells respectively, being somewhat 

 more developed than the former, less highly specialized as a rule than 

 the latter. Some authorities consider the cell membrane of bacteria 

 to be merely a concentrated external layer of endoplasm. 



The thickness of the cell membrane varies among different varieties 

 of bacteria, and it appears to be somewhat -thinner in young organisms 

 of a given variety than in the older individuals of the same kind. 

 Ordinarily it is not seen, and special stains are required to demonstrate 

 it clearly. In certain spore-forming bacteria, however, the cell mem- 

 brane is occasionally seen after the spore has matured within the cell, 

 as a thin, feebly staining shadow, outlining the original contour of the 

 organism. Bacteria which plasmolyze easily also show the cell wall 

 clearly after the cell contents have shrunken away from it. 



Capsule. A considerable number of bacteria are surrounded by 

 mucin-like envelopes, particularly when they are observed in the animal 

 body or grown in albuminous fluids. This envelope or capsule fre- 

 quently disappears when the organisms are grown in ordinary media. 

 This has led to the theory that a capsule represents an hypertrophy 

 of the ectoplasm. The significance of capsules is still a matter of 

 controversy. Two principal theories have been advanced to explain 

 the significance of capsules : according to one theory, bacterial capsules 

 are purely degenerative phenomena; the more widely accepted theory, 

 which has much evidence in its favor, maintains that capsule formation 

 is closely related to the virulence of the organisms. 1 The demonstration 

 of capsules may be an important factor in the identification of certain 

 bacteria, for example, the pneumococcus. 



Zooglea. A very few bacteria exhibit a slimy intracellular substance 

 which causes cohesion between considerable numbers of bacterial 

 cells. This intracellular substance, zooglea, is colored lightly by 

 ordinary staining methods. It is not found in any of the pathogenic 

 bacteria. 



2. Cell Substance. Cytoplasm. The cytoplasm or endoplasm of 

 living bacteria (particularly in young cultures) is usually a clear, 

 colorless, highly refractile, homogeneous appearing substance, although 

 at times various granules may be seen within it. Vacuoles also are 

 met with, usually in older bacteria. The cytoplasm usually stains 

 readily with basic anilin dyes. A few bacteria, notably B. viride and 



1 Eisenberg, Centrabl. f. Bakteriol., 1908, xlv, 148. 



