both lipoids and proteins, which are probably intermixed or combined in some characteristic manner and vary in their relative proportions in different cells, according to the specific constitution of the latter.
What are the chief peculiarities in the physical properties of these membranes, on which their physiological importance depends? Two properties appear especially significant. One of these is the semi-permeability which the membranes preserve during life, i. e., the ability to transmit water freely while holding back dissolved substances. The other is their ability to undergo reversible changes in their permeability to such.substances, either in the direction of increase or decrease. These changes of permeability may in some cells be very rapid; and there is evidence that this is especially the case with irritable tissues, and that the power of rapid response to stimuli is directly dependent on this peculiarity. How essential the semi-permeability of the plasma-membranes is to living organisms may be realized with especial clearness in the case of plants. In many of these organisms the rate of growth, the normal form and habit, and the characteristic movements and reactions are intimately dependent on the peculiar condition known as turgor, which is the expression of the outward pressure of the dissolved molecules of the cell-contents against the membranes which enclose them and which they can not pass. The diffusing molecules hence press against these membranes, often with the force of many atmospheres, and keep the cellulose cell-walls stretched and rigid. It is on this condition that the maintenance of the normal form often depends. The entrance of the water into the cell in growth is also largely due to this osmotic pressure. Thus the confinement of the molecules within the cells by membranes impermeable to their outward diffusion is an indispensable condition of the continuance of normal life-processes in these organisms. The same is true of animal cells, although here the condition of turgor is usually unimportant in itself. But, as we have already seen, the preservation of the normal protoplasmic composition in the case of any cell involves the prevention or restriction of any free or unselected diffusive interchange of materials between the cell and its surroundings. The semi-permeability found during life is the expression of the all-importance of this condition. We must therefore ascribe to the insulatory or semi-permeable character of the plasma membrane, not only the existence of conditions like turgor in plants, but even the very possibility of the existence of a stable or permanent chemical organization in any cell.
This being the case, it is not surprising to find that simple modification of permeability may profoundly modify many cell-processes. To take first a relatively simple instance: if the semi-permeability of the plasma-membrane is a necessary condition for continued life in any cell, it ought to be impossible to increase this permeability beyond a certain limited degree for any length of time without inflicting permanent in-
