272 Seifriz . — Observations on some Physical Properties of 
three does, it seems to me, give a very convincing if not irrefutable answer 
to the question. There is, first, the purely theoretical suggestion that some 
sort of membrane or septum is a physical necessity ; that a membrane 
exists wherever two immiscible liquids come in contact. The retort to this 
contention is, that this is a question of definition, and that a surface tension 
membrane has nothing in common with osmotic membranes. A second 
method of attack on the problem is through the study of permeability. 
Most workers have concluded that a ‘ semi-permeable ’ membrane about 
a protoplast is a necessary prerequisite to differential permeability. The 
third mode of approach is through morphological data based on anatomical 
evidence of the presence of a definite structural membrane. It is with this 
third method that I shall deal. 
If our discussion of the subject is to be at all intelligible it is necessary 
that we have a common understanding of the precise connotation of the 
word membrane. To the physicist a membrane is a solid structure, flexible 
in two directions and theoretically without thickness. A film, on the 
contrary, is a liquid structure, although even from physicists ( 34 ) do we 
hear of liquid membranes. Biological definitions of membranes run the 
whole gamut of physical possibilities, from a film two molecules thick to 
a tough parchment. Pfeffer, who is usually referred to as the chief advocate 
of the presence of a protoplasmic membrane, and who c has shown remarkable 
ingenuity in the development of the plasma-membrane theory ’ ( 23 , p. 282), 
was not certain of the exact nature of the membrane. In fact, in justice to 
Pfeffer, it should be pointed out that he adopted the term plasma-membrane 
‘ for the questionable surface layer of protoplasm’ ( 30 , p. 189) in order to 
have ‘a precise designation for the diosmotic layer’, but regarded the 
‘ expression plasma-membrane as a makeshift ’ which he ‘ would gladly 
drop if a knowledge of the structure and quality of the hyaloplasm would 
permit it ’ ( 29 , p. 124). To Pfeffer, then, plasma-membrane was a convenient 
term for use in the consideration of permeability phenomena. It must be 
said, however, that Pfeffer in all probability believed in some sort of 
a definite protoplasmic membrane. 1 
Von Mold ( 26 , p. 93) was one of the first to suggest that the ‘ primordial 
utricle’ was, perhaps, limited without by a special membrane lying next to 
the cell-wall. (It is interesting to note that von Mohl cautioned against 
regarding the plasma-membrane as a solid layer sharply differentiated from 
the inner protoplasm.) De Vries recognized not only an external membrane, 
to which he gave the name ‘ ectoplast ’, but also an inner protoplasmic 
membrane, which he termed ‘tonoplast ’, surrounding each vacuole. Recent 
workers such as Vonwiller ( 44 , p. 288) and Prenant ( 32 , p. 94) recognize the 
presence of a membrane around all cell inclusions, such as the nucleus, the 
1 ‘ In every case the limiting membranes determine whether or not a given substance shall be j 
absorbed ’ (Pfeffer, 31, p. 92 ). 
