PEARL, CULTIVATED. "Cultured" pearls are "natural" only as regards an outer skin averaging in thickness about three tenths to four-tenths of a millimetre. Examination of a section of a "natural" pearl shows, generally, a nucleus surrounded by a series of concentric layers of pearly nacreous matter. The con centric layers consist alternatively of Calcium Carbonate (Ara gonite), and the protein Conchiolin The nucleus varies, and may consist of sea-weed, sand, a parasite worm, or in general, anything which acts as a foreign irritant to the oyster, and which conforms to specific conditions to be described later. Experimental work on the introduction of nuclei into the oyster so as to obtain pearls, has been carried out principally in China and Japan, but the successful production of the "spherical" cul tured pearl was only obtained after assiduous research by the Japanese. K. Mikimoto commenced researches on pearl cultiva tion in 1891, working on a theory advanced by Dr. Mitsukuri (professor of Zoology, Imperial University of Japan). He inserted a small spherical mother-of-pearl bead between the body of the oyster, and the shell, and after several years found that the bead was covered with pearl secretion. Unfortunately these beads were firmly attached to the oyster shell, and therefore resembled "Blister" pearls. A wide range of materials can be used as nuclei, but mother-of-pearl was chosen by the Japanese in order that the pearl should consist entirely of oyster products. The secretion is periodic, and occurs during the warm season. The most vital portion of the oyster for pearl development is the outside of the epithelium which surrounds the mantle. This epithelium secretes from the outside normally, mother-of-pearl, and abnormally, the pearl. The determining factor in the forma tion of a pearl is not the presence of a foreign body in the interior of the oyster, but the epithelial cells in the subepidermal tissues of the mantle. These living epithelial cells are capable of secreting the nacreous matter. Nuclei which enter the body of the oyster without carrying a particle of the living epithelium will not cause the development of a pearl. In cases where the irritant or nucleus is directly attached to the epithelium of the mantle, the oyster envelopes it with a covering of epithelial cells, forming a sac, which is then coated with successive layers of nacre to produce the pearl. "Natural" pearls are found which contain no central nucleus, and Dr. Alverdes and K. Mikimoto inde pendently, by transplanting small sections of the living epithelium cells of one oyster into another, demonstrated the development of pearl matter, but such pearls were very irregular in shape, and entirely free from nucleus.
of maturity, the oysters are operated upon for pearl development. The shell of one oyster is removed, and a small spherical bead of mother of pearl is placed on the outside of the secreting epithelium of the mantle. This epithelium is then dissected from the oyster, and drawn over the mother of pearl nucleus so as to form a sac, which is ligatured at one end. The sac is next taken from the oyster, and grafted into the sub-epidermal tissues of a second oyster; the ligature is removed, and after treatment of the wound with astringents, the second oyster is placed in a cage, and finally returned to the sea. During the next seven years the oyster continues the secretion of the nacreous layers upon the mother-of-pearl nucleus, and development proceeds exactly as in the formation of a "natural" pearl. Supervision is maintained during this period, and finally the oysters are opened for the extraction of the "cultured" pearls. Approximately sixty per cent of the oysters operated upon yield pearls, but owing to the irregularity in shape, due to uncontrollable factors in their .development, those of marketable value are only about 5%.
(J. R. D.) Distinction Between Cultured and Natural Pearls.— The ordinary cultured pearl, because of its external appearance, can readily be distinguished by the expert from the natural. In the case of the finer grades, distinction was not at first easy ex cept by the cutting of a cross-section, thus disclosing the interior of the pearl. Two French scientists, Chilowsky and Perrin, in vented an instrument called the endoscope, which introduces into the holes of drilled pearls a powerful light so that the origin (whether natural or cultivated) of the pearl can be determined and the thickness of the pearly covering of cultivated pearls meas ured. A skilful and experienced operator can examine over 15o pearls in an hour. X-ray photography is now also employed successfully for the same purpose, but the process is rather slow. An instrument of German origin which makes use of power ful electromagnets has proved successful in discovering the nature of pearls which have not been drilled. Many other types of instrument have also been employed, but not so success fully. (X.) "On the Structure of Pearls and the Chinese Mode of Producing Them," Annals of Philosophy (new series, vol. ix., 1825) ; L. Boutan, "Perles naturelles et perles de culture," Annales des Sciences nat., zoologie, t. vi. (Masson, Paris, 1923) ; "Une perle fine de culture sans noyau de nacre," C.R. Acad. des. Sc. No. 8 (Aug. 21, 1922), and La Perle (Gaston Doin, Paris) ; Seurat, L'Huitre perliere (Masson, Paris) ; Lemaire, "La Production industrielle des perles fines au Japan," Bull. de la Societe franco-japonaise de Paris (xlviii., 1921) ; Lyster Jameson, "The Japanese Artificially Induced Pearl," Nature (May 5921) ; Alverdes, "Ueber Perlen and Perlbildung," Zeit. fur Wissenschaftliche Zoologie Bd. C. V. Heft. 4 (Leipzig, 1913).
(J. R. D.)