336 DR. H. LYSTER JAMESON ON 
which so often suggest parasites, are usually in the Ceylon Pearl- 
Oyster pseudo- nudlet of spheerocrystal- -like structure, each of 
which is reducible to a certain amount of amor phous non- 
calcified matter forming the lining of the nuclear cavity 
(figs. 35, 36, 37, 40, 41, & 44, am.), and the same substance, some- 
times stratified, Borne unmce not stratified, containing depositions 
of calcium carbonate. It may be necessary to differ entiate these 
pseudo-nuclei into two groups, 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 dishmnction 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 pereentage which have as nuclei grains of 
sand or other foreign bodies. These cases might cer tainly be held 
to give support to the old theory that the Saennletion 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 other hand evidence is not lacking that 
the matter is by no means so simple as this. I am instituting 
some experiments on J/ytilus (in connection with investigations 
on the life-history of the pearl-inducing Trematode that I am 
carrying on with the aid of a Goverment 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 (@) in connection with mechanical i injuries to the 
shell, as Herdman suggests, or (6) from the intestine, by perfora- 
tion or the wall, and escape of such bodies as are seen in figs. 44 
& 45 (Pl. XLIIL.) into the blood, or (ce) through lesions of 
parasitic origin. 
With regard to the last-named figures, it is interesting to note 
that so long ago as 1857 Kelaart (277) suggested that the siliceous 
skeletons of diatoms might become the nuclei of pearls. 
Professor Herdman’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 ( preparations, 
were obviously muscle-pearls, some of which are shown in figs. 9— 
12 (Pls. XXXV. & XXXVI.). In four of these preparations the 
eyst-like hypostracum- pearls were also present. 
Kight pearls which were not obviously muscle-pearls, repre- 
senting six preparations, are briefly described below. 
(a) A pearl described in the label of slide as a “ muscle- -pearl.” 
[78] 
