Die, DR. H. LYSTER JAMESON ON 
With regard to the mechanism by which the Cestode is supposed 
to cause pearl-formation, Prof. Herdman is unable to contribute 
much. He seems to recognise that the particular conditions 
necessary to transform the Cestode into a pearl-nucleus are not by 
any means universally present, and that it is only, so to speak, 
under exceptional circumstances that the Cestode, which is very 
abundant in the Ceylon Pearl-Oyster, becomes the centre of a pearl. 
The larva is surrounded by a connective-tissue cyst, and has not 
been satisfactorily demonstrated in any instance with an epithelial 
*‘ pearl-sac” (such as I described tor the Pearl-inducing 'Trema- 
tode in Mytilus), though supposed proliferations of cells inside the 
connective-tissue cyst are figured in the Report (Part V. Pearl- 
Production, pl. iii. fig. 7). These, being inside a thick fibrous 
connective-tissue capsule, are difficult to accept as being equivalent 
to a pearl-sac, which I generally find to be surrounded by the 
spongy subepidermal parenchymatous tissue, except in the case 
of those parts of a ‘ muscle-pearl” into which muscle-fibres are 
inserted. From my own observations I am rather inclined to 
regard these “cells” as granules excreted by the parasite itself, 
with possibly an admixture of wandering leucocytes. In any ease, 
if this is an epithelial pearl-sac, what becomes of the thick fibrous 
eyst outside it, which is certainly not present around the pearls ¢ 
Professor Herdman himself (see below) does not think the Cestodes 
enveloped in thick connective-tissue cysts are destined to become 
nuclei of pearls. 
The supposed migration of ectoderm-cells into the wall of a 
pearl-sac already formed and already containing a pearl, as figured 
in Part V. (Pearl-Production), pl. i. figs. 18-20, seems to be a 
matter quite apart from the question of the primary origin of the 
pearl-sac. 
On p. 23 of Part V. Prof. Herdman says :— 
“Tt is quite evident from the examination of a large series 
of sections, such as we have worked through, that the 
inajority of these encysted parasites do not become encased 
in pearls. Probably none of those in thick connective-tissue 
cysts are destined to form nuclei. They are awaiting their 
legitimate further development in the next host, after their 
sheltering molluse has been devoured by a fish. In such 
cysts and around such parasites we find no epithelial sac, and, 
as a consequence, there can be no pearl. Whether or not it 
is the case that only dead parasites supply the stimulus 
necessary to induce pearl-formation, and whether, as Giard 
has suggested, the parasites may be infested and killed by a 
species of Glugea, so that that Sporozoan comes to be even- 
tually responsible for the pearl, we are not prepared to say 
—we have found no fresh evidence in the Ceylon material 
bearing upon that point. It seems clear to us, however, that 
the epithelium is always associated with pearl-formation, and 
that in the absence of the epithelium only a thick-walled 
connective-tissue cyst is produced. If we adopt the view (see 
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