THE PERMEABILITY OF MEMBRANES 129 



on the outer surface of the cell protoplasm exists also on the surfaces of the 

 vacuoles enclosed within it. 



De Vries (1885) takes Spiroyyra and plasmolyses by immersion in 10 per cent, potassium 

 nitrate coloured with eosin. After about an hour, the cells die, become stained and the red 

 shrunken protoplast lies in a rose-coloured liquid situated between it and the cell wall. The 

 vacuoles alone remain unstained and sometimes shell out of the cell as colourless balls, which 

 slowly take up the dye. In this process it is seen that the surface layer becomes very deeply 

 stained before the dye penetrates to the interior liquid. 



At the beginning of the present section, it was pointed out that contents of the 

 protoplasm, capable of lowering surface energy, are concentrated on the surface and 

 are, in all probability, the origin of the cell membrane. The experiment just 

 described suggests a further important point. The interface between two phases 

 may be regarded as belonging to both phases, so that constituents of both phases 

 will be concentrated there if they lower surface energy. This circumstance does 

 not much concern the protoplasm of organisms like Amoeba or the cells of plants, 

 for the most part, where the external phase is nearly pure water, but is of consider- 

 able importance in the higher animals, where the fluids in contact with the cells 

 are of a highly complex composition. The difference seen in the experiment of De 

 Vries, quoted above, between the outer cell membrane, which has been killed, and 

 allows potassium nitrate and dye to pass freely, and the membrane of the vacuole, 

 which is not for some time made permeable to them, suggests that the composition or 

 structure of the membrane in contact with the contents of the vacuole is not the same 

 as that of the outer cell membrane. This difference is probably due to the presence 

 of substances in the vacuole, which contribute to the formation of the membrane. 



It will be noticed that the view here taken as to the nature of the cell 

 membrane implies that it is a variable thing as regards its composition, since 

 this depends on the substances present in the protoplasm of the cell, and in 

 the surrounding medium, at any given time. In a certain sense, it is, indeed, a 

 part of the protoplasm, so that it is not to be wondered at that its permeability 

 is capable of change with varying functional states of the cell. The fact that 

 it is readily formed is shown by the experiment of Nageli, described above, 

 where a new surface of protoplasm becomes rapidly covered with a membrane, 

 having apparently the same properties as concerns permeability as the original 

 one. That it can be reabsorbed is shown by the facts that pseudopodia of 

 protozoa will fuse together, and that a number of amoeboid organisms, as in 

 Mycetozoa, will unite to form a plasmodium. In the above sense, we may 

 accept the view taken by Hbber (1911, p. 264), that the cell membrane is a 

 living structure. In the way in which I regard it, it may be said to be a 

 local concentration of integral parts of the cell protoplasm. 



There is a certain amount of optical evidence of the existence of something on the surface 

 of protoplasm distinct from the inner mass. Gaidukov (1910, p. 51, and Fig. 3s on plate v.) 

 describes, in a germinating spore of a mycetozoon, the appearance under dark ground illumina- 

 tion of a reticulated appearance on the surface of the protoplasm ; this network had a violet 

 colour, while particles in the endoplasm had a yellow colour. Osterhout (1912, ii. p. 114), 

 also, saw an obvious change on the surface of protoplasm under the action of calcium. It is 

 well to be cautious in the interpretation of these phenomena, owing to the possibility of 

 diffraction effects. 



The question of the chemical composition of the cell membrane has excited 

 much discussion. Since lipoid substances, with cholesterol, are universal con- 

 stituents of protoplasm, while they possess in a marked degree the power of 

 lowering surface tension, it is practically certain that they must form an 

 important part of the membrane. Now, Overton (1899) has advocated the 

 view that the limiting membrane of the cell is essentially of a lipoid nature, 

 and has supported this hypothesis by a large amount of powerful evidence, 

 which it is important, as well as instructive, to examine somewhat closely. 

 It is, in the first place, a very remarkable fact that, in the case of cells of 

 the most various kinds, in the state in which they are usually investigated, 

 the substances which easily obtain entrance into the cell are just those which 

 are soluble in lipoids. In view of certain facts, to be spoken of later, it is, 

 perhaps, more correct to say, that those substances in which lipoids are soluble, 

 such as alcohol, chloroform, benzene, etc., and those which are themselves soluble 



