Choriovitelline placenta

A placenta (seen in some lower animals) in which the chorion is formed by the fusion of yolk-sac mesoderm and vessels to the inner face of the serosa.

It is formed by the yolk sac and chorion . The yolk sac gets fused with the chorion and subsequently wrinkles develop that hold the embryo to the uterine wall and the embryo, thus forming the chorio vitelline placenta . The chorionic blood vessels are connected with the vitelline blood vessel of the yolk sac . It is a primitive type of placenta developed in a few primitive marsupials like Didelphys , Dasyurus etc .

...
Wikipedia