In contrast to the few species of viviparous reptiles that develop elaborate chorioallantoic placentae and ovulate eggs with relatively low yolk content, most viviparous speciesovulate large yolked eggs and have chorioallantoic placentae that are structurally conservative. However, the placentae of the isolated yolk mass, the omphaloplacenta and omphalallantoic placenta, are sites of structural elaboration in these species. Vitellogenesis provides the primary source of nutrients for development, yet supplemental nourishment is contributed by the uterus. The embryo is dependent on the placentae for some materials, for example, gas and water exchange, whereas other aspects of placental function are facultative, i.e., the provision of some inorganic and organic nutrients, and supplement yolk resources. Embryonic nutrition in these predominantly lecithotrophic species is characterized by features shared with oviparous ancestorscombined with supplemental advantages to uterine gestation.