26 CEYLON PEARL OYSTER REPORT. 



There still remains, however, the indirect connection, the possibility that, as the 

 result of stimulation, cells from the outside of the mantle have migrated inwards to 

 surround the parasite, or that the larva destined to form the nucleus of a pearl has in 

 its wanderings carried in a few ectoderm cells which have eventually proliferated 

 around it to form the pearl-sac. These would naturally be very difficult matters to 

 prove, and the fact that no undoubted evidence of the migration, active or passive, 

 of ectoderm cells in the case of the Ceylon pearl oyster has yet been found is not 

 sufficient to disprove the possibility that the process takes place. 



As a matter of fact there are some appearances in our sections that might be 

 interpreted as indicating a migration of ectoderm cells (Plate I., figs. 18, 19, 20). 

 When the pearl is in the mantle there is no great thickness of connective tissue between 

 the ectoderm cells and the very similar epithelium forming the pearl-sac, and in some 

 cases we have observed cells giving the same appearance and staining reactions in 

 intermediate positions, and a few of these sub-epithelial cells are undoubtedly 

 undergoing division (fig. 18). These may be regarded as wandering and proliferating 

 ectoderm cells ; we think they can scarcely be interpreted in terms of the other 

 alternative to which we now pass. 



The second view that may be held is that the epithelium of the pearl-sac is formed 

 from the neighbouring mesodermal connective-tissue cells modified in sitv- in response 

 to the stimulation caused by the parasite. In favour of this view there is the absence 

 of any direct evidence of the derivation of the cells from elsewhere, so that, so far as 

 appearances go, the cells, although so very similar to those of the ectoderm, seem to 

 arise from the tissue in which the parasite and pearl are placed and in the present 

 state of opinion amongst pathologists no one is likely to deny that indifferent 

 mesodermal cells might become aggregated around a foreign body to produce an 

 epithelial sac. Whether, however, we should be justified in imagining that such an 

 epithelium might, under the stimulation of the parasite, produce layers of pearly 

 material similar to the ectodermal nacre of the shell, is not so clear. 



It is a distinct difficulty in the acceptance of this view that so many parasites in 

 all parts of the body are merely surrounded by connective-tissue cysts, have no 

 epithelial sac, and are apparently not being encased in pearls. If the pearl-producing 

 epithelium can be formed in situ from connective-tissue elements, one would expect 

 that every quiescent parasite which was becoming encysted would eventually be the 

 centre of a pearl ; but that does not appear to be the case. For one cyst pearl in our 

 Ceylon material we find something like 100 encysted parasites, and these are surrounded 

 by laminated connective tissue which may extend for several times the diameter of 

 the parasite (Plate II., fig. 19). 



Another strong reason against accepting this view is the point mentioned above, 

 that pearls in different parts of the mantle present the characters of the ectodermal 

 exoskeleton of their own neighbourhood. It would be difficult to understand that 

 indifferent mesodermal cells could simulate specialised ectodermal cells to that extent. 



