STRUCTURE AND CONSTITUENTS OF THE BACTERIAL CELL 27 
1. Cell Membrane.— Ectoplasm.— Bacteria appear to possess a spe- 
cial external boundary layer, cell membrane, or ectoplasm, which 
is rijiid and maintains the shape of the organism, (renerally speaking, 
this cell membrane is intermediate in its properties between that 
characteristic of animal and of plant cells respectively, being some- 
what more developed than the former, less highly specialized as a rule 
than the latter. Some authorities consider the cell membrane of 
l^acteria to be merely a concentrated external layer of endoplasm. 
The thickness of the cell membrane varies among ditt'erent varieties 
of bacteria, and it appears to be somewhat thinner in young organ- 
isms 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 membrane 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 
frequently disappears when the organisms are grown outside the 
animal body in ordinary media. The relativ-ely rapid disappearance 
of the capsule under such conditions has led to the theory that a 
capside represents merely 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 evi- 
dence in its favor, maintains that capsule formation is closely related 
to the virulence of the organisms.^ Kecent work, however, indicates 
that the capsular substance of the pneumococcus types, and certain 
members of the Mucosus capsulatus group confers "species specificity" 
upon these respective organisms.'- The demonstration of capsules 
may be an important factor in the identification of certain bacteria, 
for example, the pneumococcus. 
Zobglea.— A very few bacteria exhibit a slimy intracellular substance 
which causes cohesion between considerable num])ers of bacterial 
cells. This intracellular substance, zooglea, is colored lightly by 
ordinary staining methods. It is not found in any of the common 
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, more commonly, however, in older bacteria. The cyto- 
' Eisenberg: Centralbl. f. Bakteriol., orig., 1908, 45, l.'M. 
2 See Heidelberger: Chem. Rev., 1927, 3, 40.3 for excellent summary. 
