Walter Stiles 
162 
ments must be very crude approximations. With this provision Seifriz 
concludes that the thickness of the plasma-membrane is of the same 
order of magnitude as that of precipitation membranes, namely 
o-i fjL (o-oooi mm.). 
Other Cell Membranes 
Some of the arguments for the presence of plasma-membranes 
bounding the vacuole and the outside of the protoplasm hold equally 
for the presence of such membranes between the protoplasm and 
nucleus, and the protoplast and chromatophores or plastids. Kite 
(1913 a) indeed, who regards plasmatic membranes as hypothetical 
structures, regards the nuclear membrane as definitely present. He 
describes it as a stiff gel and not to be confused with hypothetical 
structures such as plasmatic membranes. Accepting for the moment 
the membrane theory of cell permeability, an experiment described 
by Osterhout (1913 b) illustrates very prettily how such membranes 
may differ in permeability from the plasma-membrane. Cells of 
the marine alga Griffithsia contain chromatophores which are pig¬ 
mented with chlorophyll and a red pigment soluble in water. Under 
normal conditions the pigment remains in the chromatophores, it 
being assumed the chromatophore membrane is impermeable to 
it. If the cell is treated with certain concentrations of ammonium 
chloride, the vacuole contracts while the outer surface of the proto¬ 
plasm “still retains its full turgidity.” At a certain stage in this 
process the red pigment diffuses out from the chromatophores but 
remains within the protoplasm, which thus becomes stained deep red. 
This is held to show the differential permeability of the chromato- 
phore-membrane and of the plasma-membranes to the red pigment. 
It must not be forgotten that it would be possible to explain the 
phenomenon on the basis of changes in colloidal complexes without 
the invocation of membranes with differential permeability. 
