98 PEARLS [CH. 



note here that objections of remarkable force were 

 made to it by von Hessling, who wrote in 1858. 



The importance of his statements, however, is only 

 just being recognised. Yon Hessling believed that in 

 rare cases foreign bodies of various kinds getting into 

 the mollusc might provoke pearl formation, but he 

 regarded as the chief cause some sequence of events 

 intimately connected with the phenomena of shell 

 growth. He makes, in this connection, the very note- 

 worthy statement that the nucleus of pearls is very 

 often formed of granules of the substance ichicJi 

 fo7ins the external layer of the shell — the horny 

 periostracum. 



Reference will be made to this observation later, 

 and it will then be seen how closely this agrees with 

 some recent views. The modern period of activity in 

 pearl research may for our purpose be taken as dating 

 from about 1897. In 1899, Leon Diguet described the 

 pearl sac, already seen by von Hessling, and this Avas 

 supposed to secrete nacre round the remains of a 

 parasite which then formed the nucleus of the 

 pearl. 



In 1901, Raphael Dubois published a paper on 

 pearl formation in Mytilus (the common edible 

 mussel), his specimens being taken from the French 

 coast, and during the same year Jameson followed 

 Dubois in working at the same problem at the same 

 place — Billiers in Brittany. 



