THE STRUCTURAL BASIS OF THE BODY 25 



cell, and only these dyes, such as methylene blue, neutral red, can be 

 used for intra vitam staining. For the same reason substances which 

 have the power of dissolving lecithin and cholesterin, such as ether or 

 bile salts, also act as hsemolytic agents, i.e. they cause a destruction 

 of the red blood cells by dissolving the superficial layer which is neces- 

 sary for their preservation from the solvent effects of the surrounding 

 fluid. 



The semi -permeability of the plasma skin can be altered by changes 

 in the saline concentration or other factors of the surrounding medium. 

 Overton has shown that, whereas a 7 per cent, solution of saccharose 

 produces plasmolysis in living cells, no plasmolysis is observed if they 

 are treated with a solution containing 3 per cent, methyl alcohol 

 plus 7 per cent, cane sugar. The superficial layer, therefore, is able 

 to dissolve a mixture of methyl alcohol and cane sugar, although it 

 has no solvent power on cane sugar in pure watery solutions. It is 

 possible that, in order to serve the nutrient needs of the cells, more 

 extensive changes may take place in the permeability of the surface 

 layer under limited conditions of time and space. There is no doubt, 

 for instance, that dextrose, to which the surface layer is apparently 

 impermeable, can yet serve as a very efficient food for the cell, and 

 one might ascribe the fact that the cell assimilates only the food which 

 it requires and no more, to such limited changes in permeability. An 

 important factor in the process of assimilation, at any rate by lowly 

 organised cells, must be the relative solubility of the absorbed sub- 

 stances in the cell and its surrounding medium respectively. When a 

 watery solution of iodine is shaken up with chloroform, the latter 

 sinks to the bottom, carrying with it the greater part of the iodine. 

 If a watery solution of organic acid be shaken with ether, the latter 

 fluid will extract the greater quantity of the acid. In no case will the 

 extraction be complete, but there will be a definite ratio between the 

 amount dissolved by the ether and the amount dissolved by the water, 

 the so-called * coefficient of partage,' depending on the variable 

 solubilities of the dissolved substance in the two menstrua. In the 

 same way a mass of protoplasm will tend to absorb from the sur- 

 rounding medium and to concentrate in itself all those substances 

 which are more soluble in the colloidal system of the protoplasm than 

 in the surrounding fluid, and this process of absorption may be carried 

 to a very large extent, if the dissolved substances meet in the cell 

 with any products of protoplasmic activity with which they form 

 insoluble compounds so that they are removed from the sphere of 

 action. It is probably by such a process as this that we may account 

 for the accumulation of calcium or silicon in such large quantities in 

 connection with the bodies of various minute organisms. 



Whereas assimilation by a living cell is ultimately conditioned by 



