What is choriovitelline placenta?

Choriovitelline placenta is a type of placenta that is found in some species of reptiles, birds, and monotremes (mammals that lay eggs). In this type of placenta, the chorion (outermost fetal membrane) and yolk sac membranes fuse together, forming a single membrane that attaches to the uterine wall. This membrane provides nutrients and oxygen to the developing embryo, as well as removes waste products.

Unlike in mammals with hemochorial placentas, where the fetal blood vessels come into direct contact with the maternal blood, in choriovitelline placenta, there is an extra layer of tissue between the fetal and maternal blood supplies, called the sub-umbilical plate. This plate helps to regulate exchanges between the fetal and maternal blood, and prevents mixing of the two blood types.

Choriovitelline placenta is considered less efficient than the placentas of most mammals, as it does not provide as much direct exchange of gases and nutrients between the fetus and mother. However, it is still an important adaptation for animals that lay eggs, as it allows for some level of direct nutrient exchange with the developing embryo.