The buds grow from the villous stems into the surrounding intervillous space and are initially established in a similar way as the stem villi. They finally form terminal villi which represent the placental barrier because they are the site of exchange between maternal and fetal blood. The placental barrier initially consists of four layers: syncytium (outside), cytotrophoblast, mesenchyme and endothelium. At the beginning of month 4, most villi lose their cytotrophoblast layer and the connective tissue that surrounds the capillaries. The placental barrier is reduced to two layers: syncytium and endothelium. The placental barrier consists of chorion and therefore human placenta is termed haemochorial placenta.