348 Sir Everard Home on the changes the egg undergoes 
Dr. William Hunter ; but in what manner the embryo breaks 
its amnion, and opens a communication with the uterus, is a 
question on which Dr. Hunter is silent : indeed it is one that 
could not be touched upon till it w 7 as ascertained that the 
ovum, when it arrived at the uterus, was completely enclosed 
in the amnion. 
Having been the first to ascertain that curious fact, and 
having arrived at that knowledge through the microscopical 
observations of Mr. Bauer, I am peculiarly gratified, that the 
same powers, exerted in another correspondent investigation, 
should have enabled me to give the solution. 
In the ovum of the hen, the rudiments of the embryo re- 
quire nourishment from one source, aeration from another. 
In the human ovum one source supplies both ; but in either 
case there is a necessity for a communication between the 
blood vessels of the embryo and the source of their blood's 
aeration ; and this is effected in exactly the same manner 
in the human species, the quadruped, and the bird. Out of 
this the embryo acquires a mode of suspension in the amnion 
which secures it from injury, both in the bird and quadruped. 
The mode is as follows : that bag, which is afterwards 
in the human species and in quadrupeds to become the uri- 
nary bladder, is enlarged to a certain size with such rapidity, 
that it bursts the amnion, which is prepared to be so ruptured, 
and the arteries lying upon its two sides are carried directly 
into contact with the chorion, and there the placenta is formed 
in the open space between the two edges of the chorion. This 
is exactly similar to the vesicle in the chick bursting the co- 
vering of the yelk, and forming the enveloping bag. It is 
deserving of remark, that in the human ovum, and that of 
