FORMATION OF PEARLS 3 



in order to protect itself, secrete a pearly coat around 

 it. But valuable pearls are not those which are par- 

 tially or wholly fused with the shell, but those which 

 lie deep in the tissues of the body; and they are prob- 

 ably formed in the following manner : The intrusive, 

 irritant body forms a pit in the outer surface of the 

 mantle ; this pit deepens, and at first remains con- 

 nected with the outside by a pore ; ultimately the pore 

 closes, and the bottom of the pit becomes separated as 

 a small sac free from all connexion with the outside. 

 The sac now sinks into the tissues of the oyster, 

 enclosing in it the foreign body. It will be noticed 

 that the inside of the sac is lined by and is derived 

 from the same tissue or epithelium as covers the 

 outside of the mantle. Now this epithelium continues 

 to do what it has always been in the habit of doing ; 

 that is, it secretes a nacreous substance all round the 

 intrusive particle. Layer after layer of this nacre is 

 deposited, and thus a pearl is formed. At first the 

 layers will conform roughly to the outline of the 

 embedded body, but later layers will smooth over 

 any irregularities of the nucleus around which they 

 are deposited, and a spheroidal or spherical pearl is 

 produced. If the irregularities are too pronounced, 

 an irregular pearl is formed ; and such pearls, 

 on merely aesthetic grounds, command a lower 

 price. 



It is thus clear that pearls are formed around in- 

 trusive foreign bodies ; and until comparatively 

 recently these bodies were thought to be inorganic 

 particles, such as grains of sand. Recent research 

 has, however, shown that this is seldom the case, and 

 that as a rule the nucleus, which must be present 

 if a pearl is to be formed, is the larva of some highly- 

 organized parasite whose life-history is certainly com- 

 plicated but as yet is not completely known. The 

 knowledge,, however, which we already possess 



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