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IV. The Croonian Lecture for 1826. By Sir Everard Home, 
Bart. V.P.R.S. 
Read November 16 , 182 6 . 
The subject of the present Lecture, is an enquiry into the 
mode by which the propagation of the species is carried on, 
in the common oyster, and in the large fresh-water muscle. 
Aided by Mr. Bauer's microscopical observations, illus- 
trated by his representations of the facts that were ascertained, 
I have been enabled to lay before the Society many curious 
particulars respecting self- impregnating animals, which 
could only be brought to light by an examination of the 
organs of generation in the field of the microscope ; and 
without the continuance of his assistance, I confess myself 
unable to prosecute the enquiry. 
It is now a period of five years since we entered upon the 
present investigation, continuing it during the breeding seasons 
of these two species of bivalves. Having at last brought our 
labours to a satisfactory conclusion, I shall now detail the 
observations we have made. 
The singular fact of pearls having their origin in the abor- 
tive ova of these bivalves, has been already recorded in the 
Philosophical Transactions. Before that discovery was made, 
a pearl was imagined to be the nacral covering which the 
animal has a power of secreting upon any extraneous body, 
introduced by accident, or otherwise, between the shells, to 
render its surface equally smooth and polished with the shell 
