THE PEARL
	entering the shell of a pearl oyster is at once enveloped or washed in the creature's exuda­tions ; that the organic matter of the secretions forms a filmy envelope in which the mineral contained in them is precipitated or crystal­lizes in wave-like layers of crystals of great tenuity, and that as these layers harden the process is repeated, and that during the process the creature either revolves the object, or about it, as it is free, or fastened to the shell. It is also possible that changes in the organic matter interwoven with the calcium carbonate may produce some chemical action resulting in the crystallization of the lime, and the crystalliza­tion in turn be provocative of another deposit, each process in turn being almost simultaneous and that the process is continued until a paucity of mineral in the exudations induces a rest for recuperation, after which the process is repeated, the result being a succession of composite skins as we find them. Whatever the cause, it is evident in all parts of the shell and in the pearl that continuity of construction is periodically arrested to be resumed upon exactly the same plan, except that the material used in the suc- 168