38 INTRODUCTION 



greater than that inside and the protoplasmic membrane will be 

 forced away from the cellulose wall, while its central cavity 

 shrinks and perhaps disappears entirely, the protoplasm forming 

 a ball in the center. This is practically what occurs when a 

 plant stem is cut and it " wilts " the water is removed by evap- 

 oration, the osmotic pressure outside the cells becomes greater 

 than that inside, and the water passes out. Likewise when a 

 plant cell dies the turgor is lost because the membrane becomes 

 permeable, and so pressure soon becomes the same on both sides 

 of the cell wall. 



In animal cells the wall is not so highly developed as in 

 plants, nor is it backed up by a rigid material like cellulose ; 

 indeed, for many animal cells there is no well-defined wall and 

 the protoplasm appears to be naked. Nevertheless the behavior 

 of the animal cells indicates that they do possess what resembles 

 a cell wall, in that they behave when in solutions as if they 

 were surrounded by a diffusion membrane. The degree to 

 which phenomena of this nature are shown varies with different 

 cells ; with red corpuscles, for example, the osmotic pressure 

 influences are very marked, as shown by the wrinkling or 

 crenation of the corpuscles when they are placed in fluids of 

 higher concentration than the blood plasma, and by their swell- 

 ing and disintegration with escape of the hemoglobin (hemolysis) 

 when they are put into distilled water or solutions of less con- 

 centration than the plasma. Other tissue cells seem to undergo 

 more or less alteration from changes in the osmotic pressure in 

 the fluids surrounding them. The diffusion membrane that 

 surrounds the cell is generally not well defined, and for most 

 cells seems to be but a surface condensation of the protoplasm, 

 perhaps formed through the effects of surface tension. The 

 diffusion within the cell, however, seems to be so much more free 

 than it is through the cell wall that it is probable that the sur- 

 face layer of the cell is quite different from that of the rest of 

 the cytoplasm. It seems probable that this surface diffusion 

 membrane contains a large proportion of cell lipoids, i. e., 

 cholesterin and lecithin (for the red corpuscles this is practically 

 certain) ; hence substances soluble in lipoids penetrate the cell 

 readily, while to substances insoluble in lipoids the cell is nearly 

 or quite impermeable (Overton). Probably the wall of the 

 animal cell is not so nearly semipermeable as is that of the 

 plant cell, for nowhere in the animal body do we get such 

 turgor in the cells as we see in plant tissues. Lacking a cellu- 

 lose wall, animal cells could not develop such an internal 

 pressure without rupturing, and such a process of rupturing 



