Placentation in bats and a ruminant. (A) Hemodichorial placenta of little brown bat, Myotis lucifugus (Vespertilionidae). There is an outer layer of syncytiotrophoblast (SYN TR) and an inner layer of cellular trophoblast (CYTO TR). Note the subsurface intrasyncytial spaces or bays (ISL) (B) Endotheliochorial placenta of funnel-eared bat, Natalus sp. (Natalidae) in late gestation. The interhemal region contains enlarged maternal endothelial cells (MAT ENDO) and cytotrophoblast (CYTO TR). (C) Hemomonochorial placenta of the Mexican free-tailed bat, Tadarida brasiliensis (Molossidae). The maternal blood spaces (MBS) are lined by cytotrophoblast (CYTO TR). (D) Epitheliochorial placenta of the sheep. The endometrial connective tissue has "decidual" cells whilst the fetal component comprises cellular trophoblast (T). Fusion of fetal and maternal cells gives rise to a syncytium (S). Therefore this type of placenta is sometimes referred to as synepitheliochorial. Reprinted from reference  with permission.