ABSTRACT
Smooth, or avillous, chorioallantoic placental areas occur temporarily in early gestation in some species, but in later gestation are supplemental to villous or labyrinthine areas. Vascular mesodermal allantoic villi are the basic framework of all definitive chorioallantoic placentae of Eutheria, whether villous, trabecular, or labyrinthine in pattern. These mesodermal villi are covered by trophoblast.
