392 LIFE : OUTLINES OF GENERAL BIOLOGY 



important characteristics of protoplasm. We have seen that a dye 

 may diffuse freely within the cell, and that the ions of salts are 

 similarly free ; this is a definite indication that the many constituents 

 of living matter which are soluble in water are prevented from being 

 washed out of the cell by the presence of a membrane at the surface, 

 through which they cannot pass. This property, it must be noticed, 

 is seen not only in whole cells, but in isolated drops of protoplasm ; 

 a surface newly formed by injury quickly acquires a protective film. 

 It is also reasonably certain that some at least of the various inclu- 

 sions in the protoplasm, especially vacuoles, and probably the 

 nucleus and the mitochondria, are bounded by somewhat similar 

 intra-cytoplasmic films. 



Microscopic examination and micro-dissection give us some 

 information about these surface films, but let us consider first the 

 more patent features of the surface of various kinds of cells. In the 

 first place, there may be a wall or pellicle outside the cell, which 

 does not form part of the living matter. Plant cells, for example, 

 are characteristically confined within a firm wall or porous box of 

 the complex carbohydrate cellulose. Many of the Protozoa, such as 

 Paramoecium, and many ripe egg-cells, are invested with a pellicle 

 of some unknown material outside the living matter; in a few 

 cases (e.g. Amoeba verrucosa), it may be remarkably firm and tough. 

 On the other hand, there may be within the boundary of the living 

 matter a zone of protoplasm different from the central cytoplasm 

 of the cell; this is best seen in some unfertilised eggs (especially of 

 the starfish) and in Paramoecium and other Protozoa. This cortex or 

 ectoplasm, as it is called, is jelly-like and much stiff er and more 

 solid than the central protoplasm, from which it also differs in 

 certain physiological details. The actual protoplasmic surface film 

 is distinct from both the external and internal elements described, 

 and lies between them, at the boundary' of the living matter. It 

 cannot be seen, under the microscope, to have any apparent thick- 

 ness, yet it is firm enough to resist gentle manipulation ; while if it 

 is slowly torn it forms again with sufficient rapidity to heal the wound 

 at once. But if the film is sharply torn with the micro-dissecting 

 needle, its delicate structure is interfered with and collapses not 

 only at the point of injury, but presently round the entire cell: 

 whereupon the contents of the cell diffuse out into the surrounding 

 water and are scattered. The strength of the film varies in different 

 cells and under different conditions. 



The general interpretation of this film is that it forms a semi- 

 permeable membrane; that is, a membrane through which some 

 molecules can pass while others cannot. An artificial membrane of 

 copper ferrocyanide, for example, is permeable to water, but not to 

 cane-sugar molecules in solution in the water; although the explana- 

 tion of this property is still unknown. In many ways the cell mem- 



