60 PEARLS [CH. 



mussel (Mytilus) have often very large nuclei. The 

 pearls from the Ceylon pearl oyster on the other 

 hand have small nuclei. The nucleus is an extremely 

 important body because of the part played by it in 

 the origin of pearls. 



It is, moreover, around the question of the nucleus 

 that the various theories of pearl formation have 

 been built up. 



The nucleus, in fact, is supposed by its presence 

 to have stimulated the shellfish, so that it secreted 

 the various layers of shell-like substance around it 

 and thus formed a pearl ; or, more correctly, to have 

 caused the formation of a little sac of cells which 

 poured out these same substances. The nucleus 

 may be a foreign body or it may actually be one of 

 the substances found in the shell layers. We have 

 seen for example that in many pearls the nucleus is 

 a substance like the periostracum or the hypostracum 

 of the shell. 



In other cases, however, the nucleus is a micro- 

 scopic animal parasite, a young larval worm which 

 has penetrated into the body of the shellfish and died 

 because of its encasement in a tomb of pearl. Sand 

 grains and other hard fragments from the external 

 world may also be found as the nuclei of pearls. 



The composition of pearls must vary according to 

 whether they consist of periostracum or of the other 

 more calcareous layers. Leaving out of the question 



