24 PHYSIOLOGY 



water outwards so that the primordial utricle shrinks (Fig. 7). On 

 immersing the cells in distilled water, water passes into the cell sap 

 until the further expansion of the protoplasmic layer is prevented by 

 the tension of the surrounding cell walls. This behaviour can be 

 explained only on the assumption that the protoplasm is impermeable 

 both to sugar and to salt, but is freely permeable to molecules of water, 

 i.e. it behaves as a semi-permeable membrane. Similar experiments 

 can be made on animal cells. The most convenient for this purpose 

 are the red blood corpuscles. These also shrink when immersed in 

 salt solutions with a greater molecular concentration than would 



234 

 FIG. 7. Vegetable cells, showing varying degrees of plasmolysis. (DE VRIES.) 



correspond to the plasma of the blood from which the corpuscles were 

 derived, whereas if placed in weak salt solutions or distilled water they 

 swell up and burst, discharging their haemoglobin in solution into the 

 surrounding fluid. By comparison of various salts it is found that 

 the strength of each salt solution which is just necessary to cause 

 plasmolysis or haemolysis, as the case may be, is determined entirely 

 by its molecular concentration, i.e. a decinormal solution of sodium 

 chloride will be equivalent in its effects on the cells to a decinormal 

 solution of potassium nitrate or of potassium chloride. The imper- 

 meability of the plasma skin does not apply to all dissolved substances. 

 Overton has found that, whereas this layer is practically impermeable 

 to salts, sugars, and amino-acids, it permits the easy passage of mon- 

 atomic alcohols, aldehydes, alkaloids, &c. All these substances are 

 more soluble in ether, oil, and similar media than they are in water. 

 The passage of dissolved substances through a membrane wetted by 

 the solvent depends on the solubility of these substances in the mem- 

 brane, and Overton therefore concludes that the superficial layer of 

 protoplasmic cells must itself partake of a ' lipoid ' character, and that 

 cholesterin and lecithin probably enter largely into its composition. 

 Thus only those aniline dyes which are soluble in a mixture of melted 

 lecithin and cholesterin have the property of penetrating the living 



