Figure 1: Placental variation among mammals. (a) Gross placental shapes in different animals. Based on the gross shape of the placenta and area of contact between fetal and maternal tissues, there are four major types of placenta: diffuse (e.g., pigs and horses), cotyledonary (e.g., ruminants), discoid (e.g., primates and rodents), and zonary (e.g., dogs and cats). This image was modified. (b) Degrees of fetal trophoblast invasiveness in different types of placenta. Based on the maternal tissue in contact with the chorionic epithelium of the fetus, there are three types of placenta: epitheliochorial (fetal chorionic epithelium is in contact with the maternal endometrial epithelium), endotheliochorial (fetal chorionic epithelium is in contact with the maternal endothelium), and hemochorial (fetal chorionic epithelium is bathed in maternal blood). The red arrows and words indicate the maternal tissue directly in contact with the chorionic epithelium of the fetus in different types of placenta. MV: Maternal vessel; MMe: maternal mesothelium; FMe: Fetal mesothelium; FV: Fetal vessel; Tr: Trophoblast; Epi: Maternal epithelium; Fen: Fetal endothelium; FB: Fetal blood; Men: Maternal endothelium; MB: Maternal blood; Cyto-Tr: Cytotrophoblast; Syncytio-Tr: Syncytiotrophoblast; Syntr-I: Syncytiotrophoblast I; Syntr-II: Syncytiotrophoblasts II; STGC: Sinusoidal trophoblast giant cell.