What allows substances to pass back and forth between the placenta and the embryo?

the fetus.

Explanation: The fetus is connected to the placenta by a tube called the umbilical cord. The cord contains two arteries and a vein. Substances pass back and forth between the mother's and fetus's blood through the placenta and cord.

between the mother's and fetus blood through the placenta and cord

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.