Which structure contains intertwined capillaries from the mother and the developing embryo?

The structure that contains intertwined capillaries from the mother and the developing embryo is PLACENTA.

Explanation:

Placenta is a structure composed of both embryonic and maternal tissues. It supplies nutrients to the developing embryo.

Placenta is a unique organ found mainly in mammals, that allows developing fetus to have all the nutrients from their mother's blood stream even before they are born.

Besides providing the nutrients, placenta also provides attachment of fetus to the mother's Uterine wall and  allow the fetus to transfer all the waste products to the mother's blood for excretion.

Placenta composed of 2 parts : The maternal part called as Decidua Basalis and the fetal part called as Chorion Frondosum. The parts are held together by Anchoring Villi.

Maternal blood flows from the maternal circulatory system through the intervillous space (space surrounding the chorion)  nutrients and other exchange occurs and blood flows back in the maternal circulation.