294 DR. H. LYSTER JAMESON ON 
epidermal sac (/. ¢c. pl. xv. fig. 5) of the same nature as the outer 
shell-secreting epidermis. In such a case it is easy to understand 
how the parasite, when it dies, becomes encased in a pearl, laid 
down, layer upon layer, by this epithelium *. 
(7) MAvTERIALS AVAILABLE FOR THE PRESENT INVESTIGATIONS. 
I have throughout these investigations been seriously handi- 
capped by the extreme difficulty of obtaining material. Many of 
the points which remain obscure could probably be cleared up if 
IT could obtain properly preserved specimens of pearl-bearing 
oysters from the Gulf of Manaar. Unfortunately, I have been 
quite unable to obtain these. 
I endeavoured to do so through the Ceylon Company of Pearl 
Fishers, Ltd., and Mr. Southwell, but without success, Mr. South- 
well replying that there were no oysters on the banks and that 
his own preserved material was finished. The Company, however, 
kindly forwarded to me a suggestion made by Mr. Southwell to 
the following effect :— 
“As it is probable Dr. Jameson requires Ceylon pearls 
(with the particular parasite giving rise to same), I would 
suggest that in order to ensure that the pearls are from 
Ceylon that they be bought here. I shall be glad to pur- 
chase pearls for Dr. Jameson, if he will give me some idea 
what to get and how much to spend.” 
J gladly availed myself of this offer, and asked Mr. Southwell 
to spend five pounds in the purchase of ‘“ cyst-pearls.” For this 
sum he procured from a local jeweller a parcel of 21 small ‘ fine” 
pearls, which I received in February 1911. 
* T must here incidentally refer to a quite erroneous interpretation which was 
placed upon the expression of my views as to the origin of the sac in Mytilus in my 
paper above referred to. 
My account of the development of this sac on p. 149 appears to have been taken 
by Herdman and by Boutan (3 & 4) toimply that I thought the sac arose from the 
mesoblastic connective-tissue elements of the mantle. As I explained in a letter to 
Prof. Herdman, which he was good enough to publish, as showing my views, on 
p. 9 of Part V. of his Report, I never had any doubt that the sac was a true 
epidermis. What I wished in my paper to emphasize was that in Mytilus it 
appeared to arise independently of, and not in continuity with, the outer epidermal 
epithelium, perhaps from in-wandering epidermal cells, perhaps from more deeply 
seated elements of epiblastic origin, some of which (e.g. certain flask-shaped glands in 
Margaritifera, see Pl. XLI. fig. 33) appear to project below the basement-membrane. 
Had I dreamed that I should have been suspected of attempting to promulgate 
heretical views on the doctrine of the immutability of the three primary germinal 
layers, I would have been more cautious in the choice of my phrases. But even if 
my wording in that paper was unintentionally somewhat ambiguous, my résumé of 
my work in ‘ Nature’ (26) should have cleared away any misconception, for in that 
paper I definitely stated (p. 280) that “a true pearl is laid down in a closed sac of 
the shell-secreting epithelium, embedded in the subepidermal tissue of the mantle 
and completely cut off from the outer epithelium itself..... Such a sac, with its 
contained pearl, may be compared to a human atheroma cyst.’ I have not yet 
reached the stage at which I can add to what I said in 1902 about the actual mode 
of origin of the epidermal sac in Mytilus, but I hope before long to be able to 
contribute some more facts on the subject. 
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