534 



THE MICROSCOPE. 



It is now, however, very generally admitted to be a dis- 

 eased condition. Pearls are matured on a nucleus, con- 

 sisting of the same matter as that from which the new 

 layers of shell proceed at the edge of the Mussel or 

 Oyster. The finest kinds are formed in the body of the 

 animal, or originate in the pearly-looking part of the shell. 

 It is from the size, roundness, and brilliancy of pearls 

 that their value is estimated. 



The microscope discloses a difference in the structure of 

 pearls : those having a prismatic cellular structure have a 

 brown horny nucleus, surrounded by small iinperfectly- 



Fig. 249. 



1, A transverse section of a Pearl from Oyster, showing its prismatic structure. 

 2, A transverse section of another Pearl, showing its central cellular struc- 

 ture, with outside rings of true pearly matter. (Magnified 50 diameters.) 



formed prismatic cells ; there is also a ring of horny 

 matter, followed by other prisms, and so on, as represented 



