276 Sir Everard Home on the production 



upon comparing the size of the central cell with that of one 

 of the ova, it is exactly large enough to contain it. The ova 

 themselves are formed upon pedicles in the same manner as 

 the yelks of the pullet's eggs, and must, when completely form- 

 ed, have a similar mode of being discharged. 



From these facts, I have been led to conclude, that a pearl 

 is formed upon the external surface of an ovum, which, hav- 

 ing been blighted, does not pass with the others into the ovi- 

 duct, but remains attached to its pedicle in the ovarium, and 

 in the following season receives a coat of nacre at the same 

 time that its internal surface receives its annual supply. 



This conclusion is verified by some pearls being spherical, 

 others having a pyramidal form, from the pedicle having re- 

 ceived a coat of nacre as well as the ovum. 



It is the nacral shining lining' of the central cell that pro- 

 duces the lustre peculiar to the pearl, which cannot be given 

 to artificial ones. 



Pearls being composed of concentric layers of nacre, which 

 are annual, must be of slow growth, and those of large size 

 can only be found in full grown oysters. 



After making these observations, Sir Everard candidly quotes 

 from the Philosophical Transactions 1674, No. 101, p. 11, 

 a similar explanation of the origin of pearls, as given by Christ. 

 Sandius, who had it from a Dane, Henry Arnoldi, who had 

 observed the facts himself, at Christiania in Norway. The fol- 

 lowing is the passage alluded to. 



" The pearl shells in Norway, and elsewhere, breed in fresh 

 water. Their shells resemble those commonly called muscles, 

 but they are larger. The fish in them looks like an oyster, 

 and it produces a great cluster of eggs like those of craw-fish, 

 some white, some black, which latter become white, the outer 

 black coat being taken off. These eggs are cast out when 

 ripe, and then grow, becoming like those that cast them. 

 But sometimes it happens, that one or two of those eggs stick 

 fast to the sides of the matrix, and are not voided with the 

 rest. These are fed by the ovster against its will, and they 

 grow, according to the length of time, into pearls of different 

 sizes, and imprint a mark both in the fish and the shell." 



This quotation we have made from Hutton Shaw and Pear- 



