GENESIS OF PEARLS
In
the first place all these parts of the mantle which supply the material
for the epidermis, the middle shell, and the lining, are enclosed
within the shell and in touch with the lining yet each receives the
exudations of that part of the mantle which supplies the material
suitable for it, the mantle invariably pushing the coarser excretions
outwardly to the shell's exterior. Again, whatever the quality of the
skin of the pearl may be, it is never of conchiolin like the outer
epidermis and though sometimes similar to the plates, of which the
conchiolin is the exposed fringe, it always contains sufficient nacre
to render the surface smooth. The fact that the skins of a pearl do
sometimes correspond with the different parts of the shell, and that
the same skin on the surface is occasionally partly nacreous and
unnacreous, in connection with the variation of quality which exists in
the internal composition of the skin, favors an idea that the mixed and
variable quantity of nacre in the skins may be caused by the abnormal
position of the mantle wrapped about the growing pearl which would
thereby come more or less under the influence of the calcite and
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