128 PRINCIPLES OF GENERAL PHYSIOLOGY 



Suppose that a mass of protoplasm, such as an Amotba, is immersed in water. 

 By the principle of Willard Gibbs, any constituent of the protoplasm which 

 lowers surface energy will be concentrated at the interface between the two 

 phases, forming already a kind of membrane. Further, as shown by Kamsdm 

 (1904) and described on page 55 above, many of the substances present in cells, 

 especially the proteins, suffer a kind of coagulation when subjected to such 

 concentration. Now, substances of a fatty nature, the so-called lipoids, such as 

 lecithin, and the fats themselves, are normal constituents of cells and, as we saw 

 in Chapter III., have a particularly powerful action in decreasing surface energy 

 and will naturally take a large share in the formation of a membrane of the kind 

 in question. 



An interesting experiment by Nageli (1855, i. pp. 9 and 10), discussed also by 

 Pfeffer (1897, i. p. 92, and 1877, p. 127, etc.), shows that such membranes are 

 formed on any free protoplasmic surface. A root hair of Jlydrochari* (a water 

 plant with relatively long root hairs, which are processes of the root cells them- 

 selves) is placed under a cover-glass in a solution of a dye, such as aniline-blue, to 

 which the normal cells are impermeable. The root hair is then crushed by 

 pressure and, from the places where the cell wall is torn, masses of protoplasm 

 exude and form into little balls. These balls show similar osmotic phenomena to 

 those of the entire cell. The protoplasm remains unstained by the dye. Kiihne, 

 also (1864, p. 39), describes the formation of a similar membrane on protoplasm 

 pressed out from Stentor, a ciliate protozoon. Further observations will be found in 

 Pfeffer's paper (1890, p. 193, etc.). 



It seems probable that the observations of Kite (1913), in which solutions injected into the 

 substance of certain cells, so as to form vacuoles, which behaved as if surrounded by a similar 

 membrane to that on the outside of the cell, are to be explained by this formation of a surface 

 condensation at the interface between the solution in the vacuole and the surrounding 

 protoplasm. 



It may be noted here that the clear surface layer of protoplasm, noticed in 

 Amvzba, leucocytes, Mycetozoa and similar organisms, and known as " hyaloplasm," 

 also owes its origin to surface forces. When the cell changes in dimensions, as by 

 taking up or losing water, it is found that the thickness of the layer of hyaloplasm 

 does not change, so that its total volume must have altered. It is constantly 

 maintained so as to extend to a particular depth below the surface. It is not to 

 be thought that this clear layer is the cell membrane itself, to which the semi- 

 permeability is due. This is to be seen from the fact that a dye, which is unable 

 to enter a cell, is stopped before it reaches the hyaloplasm, which remains 

 unstained, like the rest of the cell (Hober, 1911, p. 59). 



The new formation of a cell membrane on fresh surfaces of protoplasm, 

 referred to in the preceding paragraphs, occurs only in the " living " state, although, 

 under certain conditions, it remains intact after the death of the cell, as shown by 

 the following experiment of Pfeffer (1877, p. 136). A root hair of Hydrocharis is 

 mounted in an isotonic solution of cane-sugar, placed under the microscope, and a 

 trace of hydrochloric acid added. The protoplasm becomes granular and opaque, 

 and its movement ceases, that is, the cell is killed. But if cherry juice or other 

 dye, to which the normal cell is impermeable, be added, it will be seen that, 

 although the cell is dead, the membrane remains impermeable, since the dye does 

 not enter. But suppose that we now replace the coloured isotonic solution by a 

 hypotonic one. The cell expands by taking up water, but, contrary to what 

 happens in the living cell, the membrane does not expand also, so that it gives way 

 at one place or another ; the defect is not made good, the dye enters and slowly 

 stains the whole of the protoplasm. One must not, however, hastily draw the 

 conclusion that this semi-permeable membrane, after the action of hydrochloric 

 acid, is the ' same thing as the natural one. The experiment -merely shows the 

 possibility of producing a membrane similar to the natural one in its properties 

 and situation. 



Under certain circumstances the existence of an actual membrane can be made 

 visible, although there is no proof that the membrane was in existence in the living 

 cell in the same state as that seen. As already said, a membrane similar to that 



