PROTOPLASMIC STRUCTURE 107 



be still further increased, crystalloids and finally even 

 water fail to pass.' This last condition is exemplified 

 in water-proof organic membranes like the external 

 skin of most animals and the membranes of certain eggs 

 (the Fundulus egg). From an elementary or purely 

 physical point of view a membrane may be regarded as 

 essentially a thin sheet consisting of a gel of the kind 

 above. Such a gel has a large surface-area in proportion 

 to its total volume, and by virtue of its diffusion-hindering 

 property it prevents or retards the transfer of material 

 (colloidal particles, molecules, ions) between the two 

 solutions which it separates. 



Connected with this diffusion-hindering property, 

 which conditions the rate of interchange and hence the 

 rate of chemical activity at the surface between the 

 solutions separated, are certain electrical properties 

 (*' membrane-potentials"), resulting from the influence 

 of the membrane on the distribution and transfer of 

 ions between the two solutions.^ These properties are 

 apparently of fundamental importance to the bioelectric 

 processes, and their conditions will be considered more 

 fully later. 



Independently of its structural density, a membrane 

 of complex chemical composition may exhibit a selective 



^ Tlie conditions may be compared with those presented by a series 

 of "ultra-filters" of graded densities, as described by Bechhold {Colloids 

 in Biology and Medicine); the permeability decreases as the density 

 increases. 



^Cf. Lewis, A System of Physical Chemistry, II (1920), 320, for an 

 account of membrane potentials. The type of equilibrium investigated 

 by Donnan, in which solutions are separated by a membrane which is 

 impermeable to some but not all of the ions, plays an important part in 

 many membrane processes, as shown especially by Loeb in his recent 

 work. Cf. Proteins and the Theory of Colloidal Behavior, Part 2. 



