c 401 : 
XXIX On the Placenta. By Sir Everard Home, Bart. 
V.P.R. S. 
Read June 27, 1822. 
Till the human ovum had been detected in the uterus, be- 
fore it became attached, we could not have ascertained that 
the rudiments of the placenta were not formed at the same 
period with the chorion, and were, therefore, a part of the 
ovum belonging wholly to the female. 
That fortunate discovery enables us now to determine that 
the placenta, and the changes in the chorion, which in some 
animals fit it to perform the functions of a placenta, depend 
as much upon the compound influence of the male and 
female, as the peculiarities of the embryo itself. 
That the umbilical arteries had their origin from the bran- 
ches of the aorta called iliacse internee ; that these passed 
along the sides of the urinary bladder to the navel, where they 
composed the navel string, has been long known to anato- 
mists ; but in what manner the amnion was broken through, 
to allow these arteries to come at the chorion, has not even 
at this day been satisfactorily explained. 
This must now be admitted to take place by the urinary 
bladder, on its first formation, bursting the amnion, in the same 
manner as the vesicle in the embryo of the bird bursts the 
membrane o^the yelk, as is shown in the microscopical draw- 
ings annexed to a Paper I recently laid before the Society. 
When it is considered that the office of the placenta is to 
supply the circulation of the blood in the embryo with ma- 
MDCCCXXII. 3 F 
