it is not true that oysters secrete in suffering what women wear as necklaces. Disease would be the very worst cradle for pearls. The idea of disease originated in a fanciful supposition of pearls being to the oyster and mussel what gall-stones and urinary calculi are to higher and more suffering animals. Réaumur, to whom we owe so many good observations and suggestive ideas, came near the truth when, in 1717, he showed that the structure of pearls was identical with the structure of the shells in which they grow. He attributed their formation to the morbid effusion of coagulating shell-material. I presume you know that shells are formed by a secretion from the _mantle_? The mantle is that delicate semi-transparent membrane which you observe, on opening a mussel, lining the whole interior of the shells, and having at its free margins a sort of fringe of delicate tentacles, which are sensitive and retractile. A microscopic examination of these fringes shows them to be glandular in structure—that is, they are secreting organs. The whole mantle, indeed, is a secreting organ, and its secretion is the shell-material: the fringes secrete the colouring matters of the shell, and enlarge its _circumference_; the rest of the mantle secretes the nacre, or mother-of-pearl, and increases the _thickness_ of the shell. Now it is obvious that the formation of pearl nacre, and of pearls, depends on the _healthy_ condition of the mantle, not on its diseases. If the mantle be injured the nacre is not secreted at all, or in less quantities. 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.