OCR Text |
Show 1902.J ORIGIN OF PEARLS. 145 altogether a much more heterogeneous layer than that applied to the inner surface of the shell, and is usually rather thicker. The connective tissue (PI. X I V . figs. 1-4 ; PI. X V . fig. 5, c.t.) is a meshwork of irregular or stellate cells with oval or spindle-shaped nuclei, and more delicate fibres the nuclei of which are relatively longer and narrower, and stain more deeply than those of the former. There is a perfect intergradation between the two kinds of cells, and their relative abundance varies in different individuals according to the condition of the gonads. The fibres are more numerous just under the epidermis than elsewhere. The blood circulates in the spaces between these cells and fibres, and in places these lacunae are enlarged to form regular blood-sinuses. Numerous blood-corpuscles (bl.) can be seen in the meshes of the connective tissue, especially under the outer and inner epidermal layers. Yellow refractive granular masses, showing traces of cellular structure, sometimes occur in the meshes of the mantle parenchyma, especially in old mussels. These may be the remains of the broken-down gonads of previous years, or groups of dead leucocytes. In the connective tissue are embedded the nerves and muscle-fibres of the mantle, and the gonads when ripe extend into it. The structure of the shell has been very thoroughly investigated by von Nathusius KSnigsborn (1877), Tullberg (1882), and Ehrenbaum (1885), to whose observations I can add nothing new. The method in which the shell is laid down is of great interest on account of the identity in structure between the substance of pearls and that of the shell. Biedermann's recent paper (1901), which is full of new and significant facts and carefully summarizes previous observations, proves beyond all question that the organic basis of the shell (conchyolin), which is present also as the basis of pearls, is a true cuticular product, secreted or excreted by the underlying epidermis of the mantle. Biedermann shows that in both Lamellibranchs and Gastropods the calcareous substance of the shell can only be deposited in such a cuticle. The cuticular conception of the conchyolin was, I believe, first propounded in this form by Huxley (1859). In sections of the decalcified shell and mantle, I find that the epithelium is generally applied closely to the conchyolin, and its cuticular outer surface is apparently directly continuous with the latter. If, during the process of fixing, the mantle has been separated from the shell, a certain amount of uncalcified conchyolin may be found attached to the epidermis. Moreover, if the mantle of a live mussel be carefully stripped from the inner surface of the shell, a delicate transparent membrane, like that which Huxley found in Anodonta, but less conspicuous, can be detected. This membrane tears away irregularly, some parts adhering to the mantle, others to the shell. This irregular tearing is a further evidence that the uncalcified membrane is in continuity with both shell and mantle. That the mantle can, however, detach itself from the inner surface of the |