VIII] THE ORIGIN OF PEARLS 95 



and a similar process follows the introduction by man 

 of small objects between shell and nacre-secreting 

 tissues. As we have seen, however, in another place 

 (p. 53) the pearly objects so formed are not really 

 pearls, but blisters. The Chinese, for example, insert 

 rows of images of Buddha between the mantle and 

 shell and allow them to remain until coated with 

 nacre. There are, however, a few cases on record 

 where free pearls have been found with sand grains 

 as nuclei, consequently we must assume as an occa- 

 sional true cause of pearl formation — the minute sand 

 grain. Probably this cause is of rare occurrence and 

 of little importance. 



Now let us look at two very important theories, 

 and first at that which regards the entrance of a 

 parasite into the shellfish as the cause of pearl forma- 

 tion. AVe have seen that the structure of pearls is 

 intimately related to the structure of the mollusc 

 shell. The latter is secreted by a layer of cells which 

 bounds the surface of the mantle. Now it must be 

 borne in mind during the following study that pearls 

 are formed inside a little sac of cells. This "pearl 

 sac " was recognised by von Hessling in 1858. It is 

 composed of a single layer of cells, which are very 

 similar to those bounding the surface of the mantle 

 against the shell (see fig. 9). In fact, just as the 

 mantle cells secrete shell substance of one kind or 

 another, so do the pearl sac cells secrete the layers 



