Their origin and formation. 1 1 5 



opened by the knife, the less probabih'ty will there 

 be of finding any Pearl. As previously stated, it 

 is within the mantle that many of the Pearls are 

 found, and the inference is, that the interior surfaces 

 of this integument secrete the fine pearly layers 

 around the nucleus of what is to become a Pearl ; 

 whilst the layers of the shell are secreted mainly by 

 the exterior surfaces of the mantle. With the first 

 layer deposited around it, the intruder becomes a 

 Pearl, and if this nucleus is of animal or vegetable 

 matter, decomposition, or probably absorption, will 

 in time leave an apparently empty space, the cavity 

 being lined and discoloured by the residue ; a hollow 

 Pearl is thus formed, just as the hollow *' blister" is 

 formed in the shell as already described. 



The oyster is not entirely dormant, and its 

 movements, together with the varying position of 

 the Pearl within its tissues, probably regulate the 

 shape assumed by the constantly accumulating 

 layers ; owing, however, to the inclined position 

 of the oyster, the tendency of the Pearl is always 

 to work downwards through the tissues, towards 

 the hinge of the shell. Small Pearls often work 

 their way into the adductor muscle, and, owing to 

 the fibres of this part being coarse and close, it is 

 almost impossible for a large Pearl to penetrate 

 them, but numbers of small ones are frequently 



