concerning Animal Impregnation. 195 
opinions that prevail. All that can be expected from an indi- 
vidual in such a case, is to add the result of his own labours 
to one side or the other, so that in the end the preponderance 
must depend on the weight of evidence. 
The experiments I have made on this simple question do not 
allow me to incline to the side of De Graaf ; for in the rabbit 
I have never found any thing in the uterus which had a regular 
circumscribed form earlier than the sixth day, and even then 
the substance was bounded by a covering so very tender, that 
it scarcely had firmness sufficient to support the figure. Be- 
fore the sixth day, I have never seen any thing but irregular 
mucus-like masses in the uterus ; but after this time the sub- 
stance has firmness sufficient to admit of preservation in spirits, 
a specimen of which I have in my collection of preparations. 
This acquisition of figure does not depend so much on a dif- 
ference of consistence, as on the formation of membranes in- 
closing this substance. These membranes when in a more 
advanced state of formation, are known by the names of cho- 
rion and amnios. The product of conception being arrived at 
this stage, may with some propriety be called an ovum, as it 
has acquired a determined figure ; but the different constituent 
parts of it are not apparent at this early period ; on the tenth 
day, in the rabbit, an opaque spot is seen in this ovum, which 
increasing daily in its bulk, progressively manifests the forma- 
tion of the foetus. 
It is a little remarkable that in the rabbit, where the term 
of utero-gestation does not exceed thirty days, a third part of 
that time should be required to make that opaque spot obvious 
to the sight, whilst the remaining two-thirds should suffice 
to complete the formation of the foetus. It appears as if it 
CCS 
