Origin and formation of Pearls. 111
receives
its annual supply." While admitting that an ovum may occasionally form
the nucleus of a nacreous concretion, we cannot admit the general
application of Sir Everard's explanation.
The
finest Pearls are found within the mantle of the mollusc, close to the
lips of the shell, or in the soft part of the oyster near the hinge of
the shell ; the worst Pearls are those found within the close, coarse
fibres of the adductor muscle. At very rare intervals they are found
loose in the shell outside the body of the oyster ; and may, when
large, get washed out of the shell and thus be lost. Lastly, Pearls are
often found imbedded more or less deeply in the shell, having in some
cases escaped from the soft tissues. It is notable that the adherent
Pearls occur almost invariably in the flat or lower valve:
occasionally, it is true, they are found imbedded in the rounded or
upper valve, but in such cases it is observed that the shell has been
lying at the bottom, in the reverse position, thus making the rounded
valve the lower one. This is by no means an uncommon occurrence.
The
Pearls found imbedded in or under the " muscular impression " are
always small, irregular, and worthless, similar to those found imbedded
in the adductor muscle itself.
Pearls are found in infinite variety of form,
