272 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-nvicleus are not by 

 any means universally present, and that it is only, so to speak, 

 under exceptional circumstances that the Cestode, which is very 

 a.bundant 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 for the Pearl-inducing Trema- 

 tode in Mytihis), though supposed proliferations of cells inside the 

 connective-tissue cyst are figured in the Report (Part V. Pearl- 

 Production, pi. 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 leather inclined to 

 regard these " cells " as granules excreted by the parasite itself, 

 with possibly an admixture of wandering leucocytes. In any case, 

 if this is an epithelial pearl-sac, what becomes of the thick fibrous 

 cyst 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), pi. 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 : — 



"It is quite evident from the examination of a large series 

 of sections, such as we have worked through, that the 

 majority 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 mollusc has been devoured by a fish. In such 

 cysts andax'ound 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 peai-l-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 pea,rl, 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 



