GENESIS OF PEARLS
not
in touch with every part of it, there is at the extremity of its
action, an unnacreous deposit, corresponding to the deposit of
con-chiolin or calcite, at the extreme edge of the shell which precedes
the nacreous layers following within and slightly back of it. As the
luster of the pearl arises from the transparency of the calcium
carbonate modified by the undulating lines formed by the edges of the
wave-plates, it may be that the lapping action of the mantle is
necessary for the regular formation and crystallization of these
plates, and that at points beyond the reach of this action, the
depositions of the mantle are therefore not pearly.
Much
is necessarily conjectural as to the modus operandi by which the shell
and the pearl are formed but the invariable tendency toward sphericity
suggests that the nucleus of a pearl, when free within the mollusk's
mantle, is not only enveloped in its exudations, but is either kept
constantly moving with a rolling motion or lapped on all sides by the
membrane which exudes upon it the nacreous material.
163
