336 DR. H. LYSTER JAMESON ON 



which so often suggest parasites, are usually in the Ceylon Pearl- 

 Oyster pseudo-nuclei of spheerocrystal-like structure, each of 

 which is reducible to a certain amount of amorphous non- 

 calcified matter forming the lining of the nuclear cavity 

 (figs. 35, 36, 37, 40, 41, & 44, «m.), and the same substance, some- 

 times stratified, sometimes not stratified, containing depositions 

 of calcium carbonate. It may be necessary to diflerentiate these 

 pseudo-niiclei into two grouj)S, typically represented by figs. 36 

 and 40-42 respectively — the one secreted in all probability before 

 the epithelium of the sac could control its deposition at all (and 

 thus comparable to a " Harting's Body"); the other secreted by 

 an already partly functional epithelium. When material with 

 pearls in various stages of formation in situ in the tissues is 

 available, this distinction may prove to have an important 

 relation to the actual processes by which the pearl-sac arises, and 

 to the difference between the nuclei of muscle-pearls and some 

 parenchyma-pearls. 



The most remarkable thing about this lot of pearls purchased in 

 Ceylon is the high percentage which have as nuclei grains of 

 sand or other foreign bodies. These cases might certainly be held 

 to give support to the old theory that the stimulation of any in- 

 trusive particle, if it occurs at the right place, may give rise to a 

 proliferation of epidermis resulting in the formation of a pearl-sac 

 and a pearl. But, on the othei" hand, evidence is not lacking that 

 the matter is by no means so simple as this. I am instituting 

 some experiments on Mytilus (in connection Avith investigations 

 on the life-history of the pearl-inducing Trematode that I am 

 carrying on with the aid of a Government Grant) which will, I 

 hope, throw some light on this question. It is idle to speculate, 

 without experiments on living molluscs, whether these particles 

 enter the tissue {a) in connection with mechanical injuries to the 

 shell, as Herdman suggests, or {h) from the intestine, by perfora- 

 tion of the wall, and escape of such bodies as are seen in figs. 44 

 & 45 (PI. XLIII.) into the blood, or (c) through lesions of 

 parasitic origin. 



With regard to the last-named figures, it is interesting to note 

 that so long ago as 1857 Kelaart (27) suggested that the siliceous 

 skeletons of diatoms might become the nuclei of pearls. 



* C. Professor Herdmart s Slides. 



Professor Herdman's preparations, illustrating pearls, which he 

 very kindly lent me, consisted (omitting a few preparations of 

 clustered hypostracum-pearls) of 24 slides, showing about 25 

 pearls in section. Of these pearls 13, representing 7 preparations, 

 were obviouslv muscle-pefirls, some of which are shown in figs. 9- 

 12 (Pis. XXXV. & XXXYl.). In four of these preparations the 

 cyst-like hypostracum-pearls were also present. 



Eight pearls which were not obviously muscle-pearls, repre- 

 senting six preparations, are briefly desci-ibed below. 



(ff) A pearl described in the label of slide as a " muscle-pearl." 



