gems, pearl, pearls, cultured pearls

THE studies of the last dozen years or so directed to the elucidation of the mechanism of pearl production, and of the pathological conditions which induce or favour it, have made it increasingly clear that the production of a pearl is not, as was formerly largely believed, the direct result of the effort of the mollusc to cover over with nacre an irritating foreign body (e.g., a grain of sand or a parasite), but rather of the presence in the subcutaneous tissues of a portion of the shell-secreting epidermis, in the form of a closed sac, which need not necessarily be associated with any foreign body. The factors that determine the presence or otherwise of such an epidermal “island” seem to differ in different species of molluscs, and in the same species in different localities. The case in which the chain of causation is most fully known is that of the pearls formed through the agency of the trematode Gymnophallus in the common mussel, Mytilus edulis, worked out by the present writer in 1902, in which case the mollusc normally surrounds the parasite, while still alive, with a closed sac composed of the outer shell-secreting epidermis; and this sac, on the death of the parasite, or on its migration to another part of the host, lays down concentric layers of the shell substance, so forming a pearl. The fact that Gymnophallus invariably becomes surrounded by such a sac, after settling down in the mussel, and that other parasites and foreign bodies do not normally share the same fate, pointed to the conclusion that the formation of the pearl-sac, and consequently of the pearl, is due to some peculiar or specific stimulation on the part of the parasite, and not to mere mechanical irritation, such as would arise from the presence of any foreign body. But the nature of this stimulation, whether mechanical (as, e.g., by the parasite habitually carrying in with it a portion of the epidermis) or toxic (i.e., resulting from an abnormal growth of epidermal cells induced by some specific secretion of the parasite and comparable to the production of plant-galls), is a question which still awaits further investigation.