Abstract
The histological and histochemical studies were made on the organic membrane-shell of Mytilus edulis, and the results obtained were compared with the properties of the periostracums and of the conchiolin matrices of mineralized layers of normal shells of Mytilus edulis and Unio douglasiae. The laminated membranes, first produced in earliest stages of regeneration, show similar basophilic properties to the conchiolin matrices in mineralized layers. These membranes are always followed by the deposition of the brown membranous layer with quite different acidophilic properties, which are almost indistinguishable from those of the periostracums of Mytilus and Unio. Near completion of the brown membranous layer, other membranes are deposited inside it. They may be homologous to conchiolin which is the precursor to mineralization of shell not only from their ultrastructural appearance but also from their characteristic histochemical and staining properties. They become calcified with the development of the organic membrane-shell. In earliest stages of calcification, polycrystalline aggregates which are arranged in nacreous, homogeneous and prismatic architectural structures are occasionally observable at random fashion in the conchiolin membranous layer. From X-ray diffraction patterns, it appears that these aggregates are respectively composed of calcite or aragonite crystals. The above-mentioned facts indicate that a restricted region of the mantle epithelium can secrete, as well as periostracum, the calcified components of all the layers of shell, according to physiological requirement.
