40 Sir Everard Home on the propagation of the 
itself, and thus prevent it from injuring the substance of the 
animal in contact with it. 
In the Hunterian collection there are many specimens, in 
which extraneous bodies of different kinds have been intro- 
duced within the shells, through holes bored for that purpose, 
while the animal was alive ; which in the course of time 
received an external coat of nacre, and bear a general resem- 
blance to the pearl ; some of these were glass beads, some 
leaden shot ; but the lustre of the pearl cannot, I believe, be 
imitated, since it is produced by the bright internal surface 
of the central cell shining through the semitransparent coats 
which are afterwards formed upon it. 
Although the ova of the oyster and fresh-water muscle 
agree in this one particular, of becoming the nucleus on 
which pearls are formed, the process gone through before 
the young is completely formed, is not the same in both 
species. 
As the oyster is more simple in its structure, from having 
no organs fitted to give it the power of loco motion, which 
the muscle is provided with, I shall take its mode of propa- 
gation first into consideration. 
In the whole range of comparative\ anatomy in which 
separate organs are developed for the three essential purposes 
of animal life, sensation, digestion, and propagation of the 
species, those organs in the oyster are the smallest, the most 
simple, and have the least to occupy them. Their mode of 
propagation will be found even more single than it is in many 
plants, and the processes that are gone through, are carried 
on in a much shorter time. 
As the following account does not, I believe, accord 
