viii] THE ORIGIN OF PEARLS 115 



Jameson examined a very large number of oriental 

 pearls and published an account of bis results last 

 year (1912) which, though written without any know- 

 ledge of Rubbel's paper, agrees in a remarkable 

 way with this author's conclusions for the fresh-water 

 mussel. This renders it extremely probable that 

 the method described above, and discovered by 

 Rubbel, is of very general application, and is the 

 chief cause of pearl formation in many other mol- 

 luscs. 



Jameson does not call the layers seen in the pearl 

 by the names of similar layers in the shell. He 

 recognises the resemblance to repair substances of 

 regenerated shell, and so uses the terms "granular 

 repair nacre," "columnar repair substance," and 

 "amorphous repair substance," for the nacre, pris- 

 matic substance and periostracum respectively. 



The formation of the pearl sac has, however, not 

 even yet been traced in the Ceylon pearl oyster. 

 From the evidence of structure Jameson concludes 

 that the tapeworm cause is of little importance in the 

 case of Ceylon pearls, and that whilst sand grains and 

 other foreign particles or organic bodies may form 

 the nucleus of these pearls, a foreign nucleus is prob- 

 ably exceptional. The factors giving rise to the pearl 

 sacs have therefore yet to be discovered. 



We would suggest as probable that eventually 

 a sequence of events similar to that observed by 



8—2 



