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. XIV. figs. 1-4 ; PI. XY. 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 naiTower, 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 paren- 

 chyma, especially in old mussels. These may be the remains of 

 the broken-down gonads of previous years, or gToups 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 investi- 

 gated by von Nathusius Konigsborn (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 aiid 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. Biedermaim shows that in 

 both Laraellibranchs and Gastropods the calcareous substance of 

 the shell can only he 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, dui'ing 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 

 irregvilarly, some parts adhering to the mantle, others to the shell. 

 This UTegular tearing is a further evidence that the uncalcified 

 membrane is in continuity with both shell and mantle. That the 

 mantle can, however, detach itself fi-om the inner surface of the 



