And this takes place within the corianic villi. So these corianic villi contain the network of blood vessels that are part of the circulatory system of that developing embryo. And notice that the corianic villi are actually surrounded by a semipermeable membrane. So note that the maternal and the fetal blood do not actually mix because we have this semipermeable membrane. So certain things are allowed to pass across the membrane but things like blood red blood cells and bacterial cells and large molecules cannot actually pass across this semipermeable placental membrane. So the placental membrane found within the coryonic villi creates a semipermeable membrane that acts as a barrier for relatively large substances such as bacterial cells and red blood cells.