concerning Animal Impregnation. 163 
be rightly decided. It becomes then indisputably necessary to 
us in the present subject, to determine what is the criterion of 
impregnation. 
That a female is impregnated when a foetus is sensibly 
formed, is so obvious to reason that no argument can be neces* 
sary to convince us of its truth. But it is important to some 
conclusions in the sequel of this paper to prove, that a female 
has conceived before there are any vestiges of a new animal. 
The test of this condition must then be sought for in the ova- 
ries; and the well conducted experiments of De Graaf, in the 
last century, and of Baron Haller and others, in the present, 
bear so forcibly on this point, that the necessity of further in- 
vestigation is in a great measure precluded. 
But, in order that I might bear evidence of its truth, I exa- 
mined with great attention the ovaries of some full grown vir- 
gin rabbits, and found, as De Graaf has represented, that 
there entered into their composition a series of cells containing 
a transparent colourless fluid. It was indispensably necessary 
here to be certain, that these rabbits had never been admitted 
to the male, lest the remains of former impregnations should 
be confounded with virgin appearances. I therefore observed 
with care not only the appearance on the surface of these bo- 
dies, but likewise examined with great minuteness the interior 
parts ; yet in none of them could I see any of those circum- 
scribed substances, which, from their yellow colour, are called 
corpora lutea. But when similar observations were made on 
rabbits that had been impregnated at different periods, and the 
traces of those corpora lutea were more or less evident, accord- 
ing to the interval of time that had elapsed ; I may then say 
that no -corpora lutea exist in virgin animals, and that when- 
Y 2 
