294 DR. H. LYSTER JAMESON ON 



epidermal sac (/. c. pi, 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) Materials 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 vip if 

 I 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 efiect : — 



" 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 hei'e. 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." 



I 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. 



* I 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 tlie development of this sac on p. 149 appears to have been taken 

 by Herdman and by Boutan (3 & 4) to implj' 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 g-ood enough to publish, as showing my views, on 

 p. 9 of Part V. of his Report, 1 never had any doubt that the sac was a true 

 epidermis. What I wislied in my paper to emphasize was that in Mijtilus it 

 appeared to arise independently of, and not in continuitj' 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 

 3£argaritifera, see PI. 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 mj' phrases. But even if 

 my wording in that paper was uninteiitionallj' somewhat ambiguous, mj^ resume 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 sJiell-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 ci/st." I have not yet 

 reached the stage at which I can add to what 1 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. 



