Chorion Frondosum
The chorion frondosum, the embryonic portion of the placenta
consists of finger-like projections or villi of the chorion
frondosum whereas the maternal portion is composed of the
intervillous pool of maternal blood from maternal, arteries &
veins.
This allows for diffusion between the membranous layers through a
supply of arterial and venous blood from maternal vessels that
pools within the villi - consequently this is called the
intervillous pool of maternal blood.