a unit of the placenta made up of trophoblastic cells, fibrous tissue, and abundant blood vessels; it is visible grossly on the maternal surface as an irregularly shaped lobe circumscribed by a deep cleft and made up of a stem villus with numerous free branch and anchoring villi; placental vessels in the chorionic plate supply the stem villus and its branches, allowing gas and metabolite exchange across the trophoblastic layer with maternal blood in the intervillous space.