Abstract
The close apposition of embryonic or fetal membranes with maternal tissue for the purpose of maternal-fetal exchange occurs in many viviparous species of cartilaginous and bony fish, amphibians and squamate reptiles. There is nevertheless a gradation with some live born embryos depending on yolk as a nutritional source (lecithotrophy) and others receiving additional nutrition that may or may not be supplied by placental exchange (matrotrophy). In a placenta, nutrients are exchanged between maternal blood vessels and embryonic or fetal ones (hemotrophic nutrition) or through uptake of maternal secretions or cellular debris (histotrophic nutrition). Exchange of respiratory gases is an important feature and the main function of the placenta in species such as viviparous snakes that are mainly lecithotrophic. There is maternal-fetal transfer of oxygen and nutrients in all mammals. Although monotremes are oviparous, early embryonic development depends on the uptake of uterine secretions. In marsupials gestation is brief and supported by a yolk sac (choriovitelline) placenta. Eutherian mammals rely mainly on a chorioallantoic placenta, but there is great variation between and within orders in placental morphology and the nature of the interhemal barrier.
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