[ MO ] 
PERMEABILITY 
By WALTER STILES 
CHAPTER VIII 
THE PLASMA-MEMBRANE 
T he presence of a layer of protoplasmic material limiting the 
general body of the cytoplasm and differing from it in per¬ 
meability properties, is regarded by many botanists as essential for 
explaining the relation of the cell to water and its differential 
behaviour towards dissolved substances in regard to their absorption. 
Such membranes are, however, not morphologically obvious in most 
cells, and some investigators dealing principally with animal cells, 
have denied the existence of such protoplasmic membranes altogether. 
An enquiry is obviously necessary here into the evidence for the 
existence of such limiting membranes. 
The question has not been simplified by a somewhat confused 
terminology. By the term plasmatic membrane or plasma-membrane 
Pfeffer (1877, 1890) clearly meant to denote a surface layer of the 
protoplasm with semi-permeable properties, but the character of 
which was not further defined. De Vries (1884 a, 1885) used the 
terms ectoplast and tonoplast for layers limiting the protoplasm 
externally in contact with the cell wall and internally in contact 
with the vacuole respectively. These terms were used by de Vries 
and later by Livingston (1903) so as to imply apparently the same 
as Pfeffer’s term plasmatic membrane. 
Again, in some cells, as for example in the plasmodia of Myxomy- 
cetes, there is a peripheral layer of the protoplast morphologically 
distinct from the internal mass of the protoplasm, in that under the 
microscope it appears clear and not granular. It is not to be assumed 
that this outer layer, the hyaloplasm, is identical with the plasma- 
membrane of Pfeffer. “A hyaloplasmic border of measurable thick¬ 
ness can hardly be identical with the plasmatic membrane throughout 
its whole extent, although the mere existence of such a border, since 
it indicates that its internal surface has the power of repelling 
granules, shows that a certain similarity must exist between its 
characters and those of a plasmatic membrane” (Pfeffer, 1900). The 
ectosarc of Amoeba and the ectoplasm of some plant and animal cells 
appear to be similar to the hyaloplasm. 
