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.