AbstractHistologic changes in extraembryonic and embryonic tissues induced by 3 or 6 μg 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin/kg (TCDD) or 80 μg 2,3,4,7,8‐pentachlorodibenzofuran/kg/day (4‐PeCDF) were studied 24 h after the last of four daily doses administered orally to C57BL/6N mice on days 10‐13 of pregnancy. Both test compounds ruptured (1) the embryo‐maternal vascular barrier in the labyrinth, which resulted in hemorrhage of embryonic blood into the maternal circulation, (2) the visceral yolk sac membrane with the embryonic blood from the vitelline vessels escaping into the uterine, exocelomic and amniotic cavities, and (3) the maternal vascular spaces of the placental periphery resulting in hemorrhages into the interconceptal space. The role of the hemorrhagic lesions in the induction of cleft palate and hydronephrosis by the two compounds remains to be investigated. The presence of embryonic nucleated erythroblasts that hemorrhaged into the maternal lacunar network allowed the identification of maternal venous channels in the placenta. It revealed that (1) the labyrinth could be tentatively divided into two caudocranially oriented zones, an arterial and a venous zone; (2) the maternal blood in the labyrinthine lacunae circulated from the arterial to the venous zone, somewhat parallel to the uterine axis; and (3) the largest maternal vessels in the center of the placenta hitherto named the “central maternal artery,” was in fact, venous. ©