Placentation in the spotted hyena (Crocuta crocuta Erxleben), with particular reference to the circulation

A study of the placenta of the spotted hyena ( Crocuta crocuta Erxleben), based on five specimens from early in the second third of pregnancy to full term, is presented. Resembling the dog and cat in the retention to term of a large allantois and a medium‐sized yolk sac, the hyena is unique among Carnivora in the possession of a hemochorial villous placenta, allied structurally more to that of New World monkeys than to the other carnivores. The intimacy of the fetal and maternal circulations is further increased by so‐called intraepithelial capillaries in the syncytial trophoblast. The hyena, unlike most carnivores, possesses no central hematoma or marginal deposit of pigment, and lacks specialized structures in the chorionic membrane. The cytotrophoblast, however, does undergo modifications associated with phagocytosis of histotrophe in the paraplacental regions and in the junctional zone of the placenta proper. The principal mode of entry of maternal blood is directly into the subchorial sinus, into which uterine arteries, carried up in a crescent of gestational endometrial tissue at the margin of the placenta, empty. Smaller arteries reach the subchorial sinus in maternal septa that traverse the entire trophoblastic zone. Major arteries do not discharge blood at the base of the placenta, but basal drainage constitutes the sole means of return of blood to the uterine veins. The phylogenetic significance of the morphogenesis and vasculature of the placenta of Crocuta is discussed.