VIII] THE ORIGIN OF PEARLS 107 



Now, sections of the Ceylon pearls in situ differ 

 considerably from those of Mytihis pearls, and in two 

 ways. The pearl sac is not nearly so distinct, and the 

 nucleus of the pearl is much smaller and looks 

 different. In several cases Herdman could not find 

 any nucleus at all. 



We have already seen that there are two kinds 

 of pearls in the Ceylon pearl oyster, "cyst" (or 

 parenchyma) pearls and " muscle " pearls, the latter 

 occurring in large numbers near the insertions of the 

 muscles. The muscle pearls were nevei' believed by 

 Herdman to have been caused by parasites. Herdman 

 and Hornell stated that it was probable that these 

 pearls were formed by the deposition of calcareous 

 matter round some minute calcareous particles or 

 calcospherules. The origin of the calcospherules 

 themselves was guessed at by Southwell, who stated 

 that they were almost certainly depositions from the 

 blood. 



From what we have already seen, we should expect 

 an epithelial sac to be present to deposit this cal- 

 careous matter. From whence does this come if not 

 brought in by a parasite ? Herdman and Hornell, 

 noticing that the muscle pearls were formed very close 

 to the surface of the mantle, found no difficulty in 

 presuming that the epithelial cells on the surface of 

 the mantle might migrate inwards to the source of 

 irritation. So much then for the muscle pearls to 



