produces them," and compares them with calculi in the kidneys of man.
But, as observed by a more recent inquirer,[12] "though they are
accidental formations, and, of course, not always to be found in the
shellfish which are known usually to contain them, still they are the
products of a regular secretion, applied, however, in an unusual way,
either to avert harm or allay irritation. That, in many instances they
are formed by the oyster, to protect itself against aggression, is
evident; for, with a plug of this nacred and solid material it shuts
out worms and other intruders which have perforated the softer shell,
and are intent on making prey of the hapless inmate: and it was
apparently the knowledge of this fact that suggested to Linnaeus his
method of producing pearls at pleasure, by puncturing the shell with a
pointed wire. But this explanation accounts only for the origin of
such pearls as are attached to the shell; while the best and greatest
number, and, indeed, the only ones which can be strung, have no such
attachment, and are formed in the body of the animal itself. 'The
small and middling pearls,' says Sir Alexander Johnston, 'are formed
in the thickest part of the flesh of the oyster, near the union of the
two shells; the large pearls almost loose in that part called the
beard.' Now, these may be the effect merely of an excess in the supply
of calcareous matter, of which the oyster wishes to get rid; or, they
may be formed by an effusion of pearl, to cover some irritating and
extraneous body." The reality of the latter theory is strengthened, if
not proved by the Chinese forcing the swan muscle to make pearls by
throwing into its shell, when open, five or six minute mother-of-pearl
beads, which, being left for a year, are found covered with a crust
perfectly resembling the real pearl. Such is one method of getting
artificial pearls. The extraneous body which naturally serves for the
nucleus, appears to be very often, or, as Sir E. Home says, always, a
blighted ovum or egg. This theory which, however, is here but partly