Their origin and formation. 115
opened
by the knife, the less probability will there be of finding any Pearl.
As previously stated, it is within the mantle that many of the Pearls
are found, and the inference is, that the interior surfaces of this
integument secrete the fine pearly layers around the nucleus of what is
to become a Pearl ; whilst the layers of the shell are secreted mainly
by the exterior surfaces of the mantle. With the first layer deposited
around it, the intruder becomes a Pearl, and if this nucleus is of
animal or vegetable matter, decomposition, or probably absorption, will
in time leave an apparently empty space, the cavity being lined and
discoloured by the residue ; a hollow Pearl is thus formed, just as the
hollow " blister " is formed in the shell as already described.
The
oyster is not entirely dormant, and its movements, together with the
varying position of thé Pearl within its tissues, probably regulate the
shape assumed by the constantly accumulating layers ; owing, however,
to the inclined position of the oyster, the tendency of the Pearl is
always to work downwards through the tissues, towards the hinge of the
shell. Small Pearls often work their way into the adductor muscle, and,
owing to the fibres of this part being coarse and close, it is almost
impossible for a large Pearl to penetrate them, but numbers of small
ones are frequently
