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PEARL.
169
Different opinions are expressed by various authors as to the formation of this product. Some insist that it is an unfruitful ovum ; others, that it is a stone which covers a wound accidentally given by the animal to itself; others, lastly, suppose that the bivalve covers with an animal secretion some small extraneous body, which may have entered its shell by chance, and this it does perhaps that it may not be injured by its sharp edges. This opinion seems to us to be the most reason­able. We can assert as a fact that, when any pearl is sawn in two, various strata are seen to succeed each other, regularly, to the primitive nut which occupies the centre, and is of quite a different nature from the thin pearly layers.
Feuchtwanger says that the Chinese string very small pearls on a thread, separating them with knots ; then they put them inside bivalves, which are taken at the suitable moment when they are open to enjoy the sun, placing them so that they- do not touch the shell. They are afterwards put into the sea, but in an enclosed place, whence they are withdrawn after many years, when the pearls are found more or less enlarged, according to the time passed, and without those spots which in others are produced by adherence to the shell.
It is thought that if a microscopic body of any form was put into a pearl oyster purposely, and that it could remain without adhering to the shell, it would serve as a nut for a pearl, which would retain its form externally.