Читать книгу The Book of the Pearl. The history, art, science, and industry of the queen of gems онлайн

70 страница из 197

While the theory that pearls are caused by the intrusion of some unusual substance has the evidence of actual demonstration in many instances, and is unquestionably true to a large extent, yet microscopic examination of some pearls suggests the theory that a foreign substance is not always essential to their formation, and that they may originate in calcareous concretions of minute size, termed “calcospherules.” As regards their origin, Professor Herdman classifies pearls into three sorts: (1) “Ampullar pearls,” which are not formed within closed sacs of the shell-secreting epithelium like the others, but lie in pockets or ampullæ of the epidermis. The nuclei may be sand-grains or any other foreign particles introduced through breaking or perforation of the shell. (2) “Muscle pearls,” which are analogous to gallstones, formed around calcospherules at or near the insertion of the muscles. And (3) “Cyst pearls,” in which concentric layers of nacre are deposited on cysts containing parasitic worms in the connective tissue of the mantle and within the soft tissues of the body.^[69]