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Show 142 DR. H. LYSTER JAMESON ON THE [Mar. 4, variance with my experience; and apparently presupposes that the Trematode can survive complete calcification, which would indeed be a very remarkable biological phenomenon. According to Dubois it is only certain pearls that, by the death of the Distomum, escape this annual disintegration and so reach greater dimensions. Von Hessling (1858), it seems, was the first to ascertain that the pearl is formed inside an epithelial sac, and he emphasized the importance of this structure. He regarded the sac as being derived from the blood-cells. This sac has been noted by Diguet (1899), who suggests that it may be due to the stimulation of a parasite. I can find no support for Diguet's view, that the formation of the pearl in this sac proceeds on different lines to those on which the substance of the shell is deposited. The "vesicle or bag of the ovum" figured by H o m e (1826, pi. xiii.) may also be this sac. Before entering upon an account of m y own observations, I wish to express m y thanks to Mr. H. H. Arnold Bemrose for kindly preparing the microphotographs which accompany this paper ; to Baron Louis d'Hamonville for much valuable information concerning the pearl-bearing mussels of Billiers; to Mr. A. Scott, of the Lancashire Sea-Fish Hatcheries, for supplying me with abundant material from the Piel mussel-beds ; and to Mr. W . Wells, Marine Superintendent at the Brighton Aquarium, for conducting experiments for me. The distribution of pearl-producing individuals of Margaritifera margaritifera L., M. maxima Jameson, Pinna nigrina Lam., Hippopus hippopus L., and Tridacna gigas Lam., in N e w Guinea and Torres Straits, suggested to m e that pearls were the result of a specific pathological condition, and that the circumstances necessary to ensure infection were present only in certain areas, often of small extent. I soon convinced myself, by a study of material that I brought home with me, that Trematodes formed the nuclei of some of the pearls in each of the above-named species, but that others contained nothing more than a few yellowish granules in the centre. The same results were obtained with specimens of Mytilus edidis, sent m e from Lancashire by my friend Mr. James Johnstone. In all cases where the pearls had been preserved in situ in the tissues, they were found to be enclosed in a sac composed of an epithelium physiologically and histologically identical with the outer shell-secreting epidermis of the mantle. This observation at once accounted for the similarity in structure between the layers of the shell and those of which a pearl is composed. The obvious conclusion was that this sac is the direct, and the Trematode the indirect, cause of ])earl-production, and that the key to the problem of the origin of pearls might be obtained by investigating the origin of the sac and its relations to the Trematode. |