2 jo Seifriz. — Observations on some Physical Propei'ties of 
instructive, yet they differ, often markedly, from the structure and 
behaviour of living protoplasm. However, where a structure cannot be- 
readily seen in the living condition, but does, because of some physical or 
chemical change, become clear when dead, we are not, therefore, justified in 
utterly disregarding the evidence based on dead material. For example, 
the presence of a protoplasmic membrane in the living state cannot be 
indubitably established, yet the presence of the dead plasma-membrane is 
in some instances strikingly evident. As evidence for support of the 
existence of a living membrane, the presence of a dead membrane is not to 
be altogether ignored. 
Not only must every precaution be exercised to avoid describing living 
protoplasm from observations on dead protoplasm, but one must also take 
care to ascertain if the substance under observation is true protoplasm or a 
modified form or a product of it, such as vacuolar sap or yolk, for example. 
Many plant cells are more than three-fourths sap with only a layer of proto- 
plasm lining the cell-wall, and the volume of some fish eggs is nineteen- 
twentieths yolk with but a thin coating of protoplasm enveloping the latter 
(before fertilization). In working on such material it is difficult to be certain 
that one is observing the behaviour of true protoplasm and not of sap or yolk. 1 
Terminology. 
While some writers condemn too precise a definition of a word it is 
nevertheless true that a very broad use of a term leaves open the possibility 
of a great number of interpretations, none of which may coincide with the 
specific one in the mind of the writer. The word ‘ membrane ’, because 
of its free and lax use, is an excellent example of the confusion which 
exists in biological nomenclature. The following are some of the expres- 
sions which have been used to refer to a single cell structure, namely, 
the plasma-membrane: ‘phase boundary’, ‘surface film’, ‘surface layer’, 
‘ ectoplast ’, ‘(vacuole) wall’, ‘ ectosarc ’, and ‘hyaloplasm’. Where the 
jumble is one so hopeless as this, it is often convenient to use any one of 
the expressions in a purely abstract sense. This is what Stiles and 
Jorgensen have done. They ( 41 , p. 533) have recently put in a ‘ plea for 
definiteness of statement and for the avoidance of semi-mystical expressions 
such as “ permeability ” or “ plasma-membrane ’V The plea for definiteness 
is a worthy one. But their solution of the difficulty which the many 
expressions for the protoplasmic membrane present, by using ‘ plasma- 
membrane ’ to mean ‘ that part of the cell which is concerned in the 
phenomenon of permeability without reference to its actual location in the 
cell’ ( 40 , p. 50), is, perhaps, permissible where one has no interest in the plasma- 
membrane as such ; it does not, however, solve the problem of the existence 
1 Some other precautions in technique are given in a recent article on Viscosity values of Proto- j 
plasm (38). 
