816 
Mr. Home on the Mode of breeding 
the fetal blood. The embryo in these animals, therefore, forms 
the intermediate link between the ova that are deposited in 
the oviduct, and hatched there, and the fetus formed in the 
uterus ; it resembles the first, in the mode by which the foetal 
blood is aerated, and the second, in the situation in which it is 
deposited, and in the mode of its being supplied with nourish- 
ment after birth. 
In quadrupeds in general, and the human species, the fetal 
blood is aerated in a very different manner, from that which 
has been described ; for although the foetal circulation sends 
a portion of its blood to the whole of the membranes in which 
the embryo is inclosed, as in oviparous animals, the influence 
of the external air is excluded, from the coats of the uterus, 
to which these membranes every where adhere. The fetal 
blood is however close enough to that in the vessels of the 
uterus, to have the air in the maternal blood communicated 
to it. 
There are three different structures of the membranes of 
the fetus for this purpose ; one of these is the whole of the 
fetal membrane next the uterus, being extremely vascular, 
as in the horse and ass ; the second, is having only particular 
portions of it loaded with blood vessels, as in the deer ; the 
third, is having only one large mass, into which the fetal 
blood vessels pass to a certain depth, beyond which is a cel- 
lular structure, filled with blood from the arteries of the uterus, 
and taken up by the uterine veins, so that in this instance, 
there is a greater mass of blood belonging to the mother, 
nearly in contact with the fetal blood, than in other animals. 
From the series of facts which has been detailed, it appears, 
that oxygen is essentially necessary for the action of the 
