136 THE POPULAR SCIENCE MONTHLY 



ous cell-membranes are to be regarded as essentially surface-films, or 

 haptogen membranes. Not only do such thin films form about the re- 

 constituting nuclei of dividing cells, but they are also deposited about 

 various cell-inclusions, and even about division-spheres, chromatophores 

 and other cell structures under certain conditions. It is well known 

 that portions of protoplasm cut off from living cells — such as egg-cells, 

 protozoa, root-hairs, etc. — exhibit the same osmotic properties as the 

 intact cells, showing that new semi-permeable membranes are quickly 

 formed at the cut-surfaces. 



The surface of contact of the living substance with its medium thus 

 becomes the seat of deposition of certain protoplasmic constituents or 

 products which form membranes, often of a high degree of impermea- 

 bility. This impermeability is a property of fundamental physiological 

 importance. Speculation on the evolutionary origin of living cells 

 usually leads to little result, but we may at least infer that the early 

 protoplasmic systems which survived and became the ancestors of liv- 

 ing organisms must have consisted in part of colloids like proteins and 

 lipoids which had the property of forming surface-films sufficiently 

 impermeable to limit or prevent free diffusive interchange with the 

 surroundings. Only systems thus isolated to a sufficient degree from 

 the surroundings could preserve the requisite complexity and constancy 

 of composition, and hence be enabled to develop the properties of so- 

 called living beings — properties which are so widely different from those 

 shown by other natural systems. The surface-films, or plasma-mem- 

 branes, of living cells at the present time are in fact typically charac- 

 terized by a remarkably high impermeability to simple crystalloid sub- 

 stances like sugars, neutral salts and amino-acids, all of which are im- 

 portant constituents of protoplasm. Zangger expresses the situation 

 concisely when he says that living cells can contain as permanent con- 

 stituents only such substances as are not free to diffuse into the sur- 

 rounding medium. The existence of this diffusion-preventing or insu- 

 lating surface-film, the plasma-membrane, is thus a necessary condition 

 of the stability of the living system and hence of the continuance of the 

 life-processes. The living condition is in fact incompatible with 

 marked and permanent increase in surface-permeability. During life 

 the semi-permeable condition is retained; on death there is always a 

 marked increase in the permeability of the plasma-membrane; the cell 

 then undergoes a ready and rapid dissolution or cytolysis, and the con- 

 stituents serve as food to bacteria. It is probable that the various 

 intracellular membranes — nuclear membranes, vacuole-membranes, 

 sphere-membranes, chromatophore-membranes — subserve a similar 

 insulating or differentiating function. Hofmeister has indeed con- 

 ceived of the protoplasm of living cells as subdivided in this manner 

 into a many-chambered system, which accordingly permits of a high 

 degree of chemical differentiation. A variety of independent processes 



