Abstract
The placental structure of cetaceans is epitheliochorial and implantation is
diffuse (Zhemkova 1967). Mossman (1987) described the fetal membranes of
cetaceans as similar to those of Tragulidae (mouse deers) and Camelidae
(camels), primarily within the later stages of development and noted that
Stump et al. (1960) also illustrated early embryos as similar to artiodactyls.
Mossman (1987) further noted that there is an early but temporary yolk sac or
choriovitelline placenta in cetaceans that is replaced later by the
chorioallantoic placenta. The epitheliochorial placenta is considered a
secondary specialization and is found in the superorder Laurasiatheria,
which was the last mammalian superorder to arise. This superorder includes
many types of placentation, but most species with epitheliochorial
placentation are found here, including cetaceans, camels, ruminants, pigs,
peccaries, hippopotamuses, horses, and pangolins (Carter and Enders 2004).