The development of the trophoblastic cords and the chorionic villi (which are maternal blood sinuses) surround these structures, along with the supplying arteries (called the maternal spiral arteries) drain into the uterine veins. The sinuses become cannulated, and the trophoblastic cords send additional projections (called the placental villi) which increase the surface area for transfer of blood and oxygen.
http://php.med.unsw.edu.au/embryology/index.php?title=File:Placenta_cartoon.jpg Accessed October 2013. Digital file.
