The placenta of humans, and certain other mammals contains structures known as cotyledons, which transmit fetal blood and allow exchange of oxygen and nutrients with the maternal blood.

In this form of placenta, the chorionic villi form a number of separate circular structures (cotyledons) which are distributed over the surface of the chorionic sac.

[3] Each cotyledon consists of a main stem of a chorionic villus as well as its branches and sub-branches.

[4] The cotyledons are surrounded by maternal blood, which can exchange oxygen and nutrients with the fetal blood in the capillaries.

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