A choriovitelline placenta is a

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

formed by the

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

and

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

. In a choriovitelline placenta, the yolk sac fuses with the chorion and, subsequently, wrinkles develop that hold the embryo to the uterine wall, thus forming the choriovitelline placenta. The chorionic blood vessels are connected with the vitelline blood vessel of the yolk sac. Because the yolk sac is formed earlier than the

allantois The allantois ( ; : allantoides or allantoises) is one of the extraembryonic membranes arising from the yolk sac. It is a hollow sac-like structure filled with clear fluid that forms part of the developing conceptus in an amniote that helps the ...

in embryo development, a choriovitelline placenta can form earlier than the

chorioallantoic placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between th ...

. All marsupials maintain a choriovitelline placenta. (However,

bandicoot Bandicoots are a group of more than 20 species of small to medium-sized, terrestrial, largely nocturnal marsupial omnivores in the order Peramelemorphia. They are endemic to the Australia–New Guinea region, including the Bismarck Archipela ...

s also have a

.) Primates do not form any choriovitelline placenta. However, this is not to say the existence of a choriovitelline placenta is a "primitive" feature: many placental mammals, including pig, horse, and ruminants, forms a choriovitelline placenta in early development before the chorioallantoic placenta forms and the choriovitelline placenta is resorbed. Rodents and some other mammals first from a choriovitelline placenta then forms a chorioallantoic placenta, but both types are maintained throughout gestation.

References

Placenta Mammal anatomy Marsupials Embryology {{Vertebrate anatomy-stub