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The most conspicuous structural element in the connective tissue of the clam mantle and gill is the amoeboid cell characterized by its large globular oval body and small nucleus, also long cytoplasmic extension and vesicular contents. The Golgi apparatus is very prominent in the amoeboid cell and is characterized by a large number of vesicles associated with its cristernae. Glycogen, .01μ in diameter, and collagen 200 Å in diameter with a 400 Å periodicity, are found within the vacuoles of the amoeboid cells. Extracellular fibrils having a 100 Å diameter and a 200 Å periodicity, are also observed in the connective tissue. From the autoradiographic studies after administration of proline-C, the fibrils are incorporating proline, which is similar to those of collagen fiber of vertebrates. Organic crystals displaying a concentric pattern which become the calcium spherulites of the shell appear to arise primarily from mineralization of the intercellular collagen-like fibrils. The evidence indicated the calcium crystals in mantle were already organized by the fibers in tissue prior to secretion from the tissue. [ABSTRACT FROM PUBLISHER]