HAYNES : NOTES ON PEARL FORMATION. 225 



a different and valueless character as compared with those formed 

 in the softer parts of the oyster ; the former being more of the 

 nature of concretions and the latter alone being cyst-pearls. 



Dr. H. Lyster Jameson, who has made the nuclei of pearls his 

 special study, says that apart horn, those of trematode origin they 

 range from diatoms and fragments of radiolarian shells to sponge 

 spicules.! He also is of opinion that " the immediate cause of the 

 pearl is not the mechanical irritation caused by the body of the 

 parasite, but rather the toxic properties of its secretions, which 

 lead to the pathological changes (formation of the tumours that we 

 call pearl-sacs) in the tissues." ^ 



In a letter to the Tim.es (7th May, 1921) Sir Arthur E. Shipley, 

 in commenting on the new Japanese culture pearls, refers to the 

 outer shell-secreting epidermis of the mantle, and remarked that 

 " should the intrusive body press on into the interior of the mollusc 

 it will in some cases carry with it a portion of the epidermis, which 

 will in time form a little cyst around it ", secretion of nacre following 

 and the formation of a pearl. It is difficult, however, to accept 

 this proposition if it is meant to cover all cyst-pearls. 



On one occasion an Australian mother-of-pearl oyster was found 

 which not only contained free pearls enough to fill a sherry glass, but 

 both shell surfaces were studded thickly with embedded pearls of all 

 sizes varying up to that of wrens' eggs. It is difficult to reconcile 

 this as probable under Sir Arthur Shipley's proposition ; and if 

 endogenous larval reproduction, as vouched for by Mr. Southwell 

 in Ceylon pearl oysters, accounted for this extraordinary amount 

 of pearl formation, the suggestion occurs that such wholesale cyst 

 formation was attributable to metaplastic action closely akin to 

 Mr. Drew's experiences in Pecten. The same comment may be 

 advanced touching the non-pearl-bearing, " fibrous," or " connective 

 tissue cysts " containing cestode larvse, which predominate so largely 

 in Ceylon oysters over the actual jDcarl-bearing epithelial or 

 ectodermal sac. 



It is not definitely stated by anyone that foreign blisters occur 

 on the upper shell of a pearl oyster, but joearl blisters certainly occur 

 there, and the fact is one of special interest. No biologist has 

 explained how a pearl that has escaped from its sac passes through 

 the inner or outer epidermis, but some new information may be 

 expected to be published shortly as to foreign bodies being jDassed 

 through the outer part of the body wall of oysters and " blistered ". 



The controversy that exists as to the identity of the parasite, or 

 parasites to which the formation of fine Ceylon pearls is to be 

 attributed is still acute. Cestode larves are found within the tissues 

 of the pearl oysters, but Dr. Jameson maintained (1912) that " no 



1 Nature, 26th May, 1921. 



2 Jameson, Proc. Zool. Soc, 1912, p. 329. 



VOL. XIV. — OCTOBER, 1921. 16 



