Beschreibung:
<jats:title>Abstract</jats:title><jats:p>The placental labyrinth of the three‐toed sloth (<jats:italic>Bradypus tridactylus</jats:italic>) was examined by electron microscopy. The material available was from two fetuses of 55 mm and 150 mm crown‐rump length (CRL). The placenta of the younger specimen was a transition form between the syndesmochorial type and the endotheliochorial type. The interhemal membrane of the 55 mm CRL specimen consisted of the following components: (1) hypertrophied maternal endothelial cells surrounding the maternal vessels; (2) a small amount of extracellular material and spindle‐shaped cells, both presumed to be of maternal origin; (3) a layer of syncytial trophoblast that had an unusual, reticulated or sponge‐like appearance; and (4) the fetal capillary endothelium. In addition, the fetal connective tissue contained hypertrophied mesenchymal cells that were characterized by abundant granular endoplasmic reticulum. The placenta of the 150 mm CRL fetus was poorly preserved, but it was possible to determine that the placenta was of the endotheliochorial type. There was a close apposition of maternal endothelium and trophoblast. The results confirmed virtually all of the light microscopic observations of Wislocki (1927; 1928b). The ultrastructure of the sloth interhemal membrane is compared to that of other endotheliochorial placentas, particularly that of the shrew, with which it shows many cytological similarities.</jats:p>