THE PEARL OYSTER. 917 



The nucleus of the Pearl may be either a grain of 

 sand, or the frustule of one of those minute siliceous 

 vegetables known as diatoms, or a minute parasite, or even 

 one of the ova of the Pearl oyster itself. Around this 

 foreign body thin layers of nacre are deposited, one after 

 another, like the successive skins of an onion, until the 

 object is completely encysted. The Pearl is formed of 

 concentric layers of carbonate of lime, of extreme tenuity, 

 but of the same general character as those composing the 

 shell. 



For a lonj>- time it was currentlv believed that Pearls 



O * 



were found only in diseased shell-fish, and to this day, in 

 some parts of Great Britain, when a Pearl is discovered in 

 a mussel or oyster, the edible part is thrown away as unfit 

 for use, while the Pearl, however valueless, is carefully 

 preserved. Hence we often find, even at the present clay, 

 that Pearls are alluded to as " morbid secretions." (_/) 



On this subject, Professor Coutance, of the Medical 

 School of Brest, has some remarks which are at once sen- 

 sible and amusing : " From the physiological point of 

 view, the oyster does nothing abnormal in producing the 

 pearl, since the nacre of the shell is formed of the same 

 substance. It does not draw from its inside any new mate- 

 rial to make the pearl, it only employs for that purpose, 

 perhaps to the detriment of its shell, a portion of the car- 

 bonated element which constitutes it, or even serves to 

 repair it. The disease of the oyster is therefore only a 

 hyper-secretion ; without doubt it is much, and we continue 

 to lament it as we commiserate a man with a cold in the 

 brain. Another observation to make is, that nothing is 



(/) " Pearls and Pearling Life," by Edwin W. Streeter, F.R.G.S. 

 (London : G. Bell & Sons. 1886.) 



