MEMBRANES AND CELL-PROCESSES 133 



from their surfaces and deposit them in the form of definite coherent 

 layers or membranes. Similar membranes may also be formed in the 

 cell-interior. Of these, the best known is the nuclear membrane. 

 Hence, in considering the general organization of the cell, cytoplasm 

 and nucleus are usually described as bounded by definite structurally 

 distinct layei:, plasma-membrane and nuclear membrane. Vacuole- 

 membranes, sphere-membranes and plastid-membranes may also exist 

 in certain cells. To all of these structures it has been customary to 

 ascribe a more or less mechanical or simply protective or isolating func- 

 tion. On the other hand, many cells show no optically distinguishable 

 membranes, either at their surfaces or in their interior; certain 

 ameboid cells and the blood-corpuscles of vertebrates are apparently 

 without membranes and are often described as "naked masses of proto- 

 plasm." Yet in such cases the nakedness is only apparent, for it can 

 readily be shown that these cells have membranes which are highly defi- 

 nite in character, but whose existence can be demonstrated only by cer- 

 tain forms of physiological experimentation. 



The membranes whose physiological role forms the subject of this 

 article are not to be identified with those more or less conspicuous lay- 

 ers separated at the surfaces of many animal and plant cells. The 

 cellulose membranes of plant cells and the various cuticular structures 

 of animal cells are dead structures, whose function is typically passive 

 and mechanical. They are to be sharply distinguished from the mem- 

 branes about to be considered, whose role is a characteristically active 

 one, and, as I believe, fundamentally important in the life of all cells. 

 These membranes are present in all living cells without exception, 

 whether a visible external layer is present or not. Thus red blood cor- 

 puscles, though typically naked cells, show by their behavior in salt- 

 solutions of varying concentration that they are bounded by a difficultly 

 permeable surface-layer which is different in its physical properties 

 from the internal protoplasm — having in fact the essential properties 

 of a semi-permeable membrane. Plant cells, like those of Spirogyra, 

 also behave in such solutions as if the surface-layer of the protoplasm 

 were semi-permeable; the visible cellulose membrane plays no part 

 whatever in the osmotic process (plasmolysis) observed under such 

 conditions, while the invisible surface-film of the protoplasm is all-im- 

 portant. Hence in the case of plant cells the conceptions of cell-mem- 

 brane— i. e., the hardened secretion of cellulose — and of plasma-mem- 

 brane — or semi-permeable surface layer of the living protoplasm — have 

 to be kept sharply distinct. It is the plasma-membrane, the most ex- 

 ternal layer of the living protoplasm, with which I shall be chiefly con- 

 cerned in the present article, and I propose to discuss briefly various 

 questions which arise in reference to this structure: what is its phys- 

 ical and chemical nature? what are the conditions of its formation? 

 and how does it influence the characteristic activities of the cell ? 



