The chorionic ridges may develop secondary ridges (which interdigitate with secondary uterine furrows) and all bear stout chorionic villi (which interdigitate with uterine villi). Villi also occur between chorionic ridges and where there are no chorionic ridges (necrotic tips excluded). The epithelium covering the tips of the ridges and villi is ____, while that covering their bases is ____ (see Fig. 33).