Sir Everard Home on the production, &c. of pearls. 339 
this, however, where the pearl has been bored is destroyed, 
and 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 
formed, 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 
having been blighted, does not pass with the others into the 
oviduct, but remains attached to its pedicle in the ovarium, 
and in the following season receives a coat of nacre at the 
same time that the internal surface of the shell 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. 
The best mode of rendering the central cell conspicuous, 
is setting one half of a split pearl in a ring with the divided 
surface outwards, and looking at the cell through a magni- 
fying glass. 
Mr. Banks, the optician, in the Strand, has succeeded in 
contriving an apparatus for this purpose, from which the 
annexed magnified drawing is taken. 
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. 
I have engaged an intelligent person, who is on board one 
