experience
is, that a nucleus is always to be found. On the other hand the
pearlers in the North West of Australia, state, that most of the Pearls
broken there have presented a small golden-coloured cavity capable of
holding a No. 8 shot. The experience of these pearlers on the general
subject, however, is much inferior to that of the jeweller.
The nucleus of the Pearl may be either a grain of sand, or the frustule of one of those minute siliceous vegetables known as diatoms, or
a minute parasite, or even one of the ova of the Pearl oyster itself.
Around this foreign body thin layers of nacre are deposited, one after
another, like the successive skins of an onion, until the object is
completely encysted. The Pearl is formed of concentric layers of
carbonate of lime, of extreme tenuity, but of the same general
character as those composing the shell.
Sir
Everard Home, a distinguished surgeon in the early part of this
century, having been led to study the structure of Pearls, came to the
following conclusion : " A Pearl is formed upon the external surface of
an ovum, which having been blighted, does not pass with the others into
the oviduct, but remains attached to its pedicle in the ovarium, and in
the following season, receives a coat of nacre at the same time that
the internal surface of the shell
