Calcification of biological tissues is usually attributed to the operation of a template mechanism or to enzymes. An electrochemical mechanism has been proposed by the author to account for calcification in the shore crab, Carcinus maenas L. and elsewhere. In the crab, study of cuticle structure suggested that calcification may arise as precipitation of lime by an alkaline reaction resulting from diffusion processes, largely outward diffusion of salt. The mechanism is probably that salt diffusion produces a potential which results in a flow of hydrogen or hydroxyl ions through the liquid phase of the cuticle (membrane hydrolysis) or of electrons through semiconducting materials in the solid phase (producing electrode action at the two surfaces). The latter, electrochemical action, in which an alkaline reaction arises by cathodic action at the outer surface and acid by anodic at the inner, appears the more likely, although the conduction mechanism may be complex.
Further evidence concerning the dependence of calcification on diffusion is now presented. The lime of the normal plaques in crab cuticle is shown to be mobile. Thus decrease of salinity on the outer side of the cuticle, which increases the normal salinity gradient across it, causes enhanced precipitation at or on the outer surface while increase of salinity on the outer side, which reduces or reverses the salinity gradient across it, causes reduced precipitation or solution at the surface and re-deposition further in. These changes occur in similar ways in both non-living preparations of cuticle and in living crabs and are attributed to changes in the positions of acid and alkaline layers by the direction of salt diffusion. Crabs kept out of water do not calcify. Lime precipitation in the cuticle is thus mainly controlled not by the nucleating properties of a specific organic template or by enzymes more or less fixed in position, but by salt diffusion. Further, crystalline deposits of lime very similar to those in the crab have been produced by precipitation of mixtures of the appropriate salts in a foreign material, cellophane. Inclusion of iron and magnesium is necessary to produce spherulites like those of the crab and even more similar forms can be produced in thin films of magnesium hydroxide gel. It is therefore not necessary to postulate any specific organic nucleating action for the production of such deposits in crab cuticle.