Villous tree model developed in this study. The branching pattern in the stem villi of the model was indicated by the centripetal and centrifugal orders at the branches, whose example is indicated by (a) and (b), respectively. (c-e) show the side views of the model: the stem villi, white; the villi except the stem villi, blue; the intervillous space, which the maternal blood passes through, blue. As (d) and (e) show, this model is between the chorionic and basal plates. The z axis of the Cartesian coordinate system is perpendicular to the chorionic plate. The size of this model is 34.8 × 34.8 × 24.5 [mm] (1200 × 1200 × 847 [pixels], 29 μm/pixel).