A little later, mesenchymal cells originating from the parietal extra-embryonic mesoblast penetrate the branching villi and give rise to a central mesenchymal core (secondary villi). At S6, blood cells and capillaries arise in this villous mesenchyme (tertiary villi). Around the end of week 3, blood vessels of the tertiary villi become connected to the intra-embryonic circulation through vessels of the chorionic plate and the connecting stalk (embryoplacental or fetoplacental circulation).