But, although pearls depend upon the healthy, not the diseased activity of the mantle, it is clear that there must be some unusual condition present for their formation; since the secretion of nacre does not spontaneously assume the form of pearls. What is the unusual condition? Naturalists are at present divided into two camps, fighting vigorously for victory. The one side maintains that the origin of a pearl is this—an egg of the oyster has escaped and strayed under the mantle; or the egg of a parasite has been deposited there; this egg forms the nucleus, round which the nacre forms, and thus we have the pearl. The other side maintains with great positiveness that anything will form a nucleus, a grain of sand, no less than the egg of a parasite. ’Tis a pretty quarrel, which we may leave them to settle. Some aver that grains of sand are more numerous than anything else; but Möbius says that of forty-four sea pearls, and fifteen fresh-water pearls, examined by him, not one contained a grain of sand; and Filippi, who has extensively investigated this subject, denies that a grain of sand ever forms the nucleus of a true pearl. Both Filippi and Küchenmeister[16] declare that a parasite gets into the mussel or oyster, and its presence there stimulates an active secretion of nacre.
There are pearls, according to Möbius, which consist of three different systems of layers, like the shells in which they are formed; with this difference, that these layers are reversed: in the shell the nacre forms the innermost layer, in the pearl it forms the outermost. Hence the qualities of the pearl depend on the shell, and on the different proportions of nacre and carbonate of lime.
Since we know how pearls are made, may it not be expected that we should learn to make them? Ever since the days of Linnæus the hope has been entertained, and it is now becoming every day more likely to be realized. Imperfect pearls have been made in abundance. The Chinese have long practised the art. They simply remove the large fresh-water mussel from the water, insert a foreign substance under the mantle, and in two or three years (if I remember rightly) they take the mussels up again, and find the pearls formed. In this way they make little mother-of-pearl Josses, which are sold for a penny each; and I remember seeing a couple of large shells in the Anatomical Museum at Munich, the whole length of which was occupied by rows of little squab Josses, very comical to behold. I was informed that a copper chain of these deities had been inserted under the mollusc’s mantle, and this was the result.
FOOTNOTES
[12] Flourens: Cours de Physiologie Comparée, 1856, p. 9.
[13] Burdach: Physiologie, ii. 245.
[14] Darwin: Origin of Species, p. 128.
[15] Isidore Geoffroy St. Hilaire: Hist. Nat. Générale des Règnes Organiques, 1860, iii. 207 sq. Broca: Mémoire sur l’Hybridité, in Brown-Sequard’s Journal de la Physiologie, 1859.
[16] See their interesting essays in Müller’s Archiv. 1856.